Harmattan: fix segfault when removing items

Looks like a case of QTBUG-18592 -- the backtrace is very similar and removing
explicit manipulation of the "current index" prevents this crash.

It was not exactly trivial to reproduce this (on-device debugging always led to
a corrupted stack, or perhaps "just" that-version-of-gdb not being able to work
with whatever-system-setup. In the end, using Scratchbox with the regular amd64
binaries has helped, and this is the backtrace:

Program received signal SIGSEGV, Segmentation fault.
0xf68a298f in QDeclarativeListView::itemsRemoved (this=0x832bd00, modelIndex=0, count=2) at graphicsitems/qdeclarativelistview.cpp:3392
3392    graphicsitems/qdeclarativelistview.cpp: No such file or directory.
        in graphicsitems/qdeclarativelistview.cpp
(gdb) bt
 #0  0xf68a298f in QDeclarativeListView::itemsRemoved (this=0x832bd00, modelIndex=0, count=2) at graphicsitems/qdeclarativelistview.cpp:3392
 #1  0xf69f8cb3 in QDeclarativeListView::qt_metacall (this=0x832bd00, _c=QMetaObject::InvokeMetaMethod, _id=30, _a=0xffffc514)
     at .moc/debug-shared/moc_qdeclarativelistview_p.cpp:485
 #2  0xf5a819ec in QMetaObject::metacall (object=0x832bd00, cl=QMetaObject::InvokeMetaMethod, idx=90, argv=0xffffc514) at kernel/qmetaobject.cpp:237
 #3  0xf5a950cf in QMetaObject::activate (sender=0x853bf30, m=0xf6acddac, local_signal_index=2, argv=0xffffc514) at kernel/qobject.cpp:3280
 #4  0xf69f6ce2 in QDeclarativeVisualModel::itemsRemoved (this=0x853bf30, _t1=0, _t2=2) at .moc/debug-shared/moc_qdeclarativevisualitemmodel_p.cpp:147
 #5  0xf6890631 in QDeclarativeVisualDataModel::_q_itemsRemoved (this=0x853bf30, index=0, count=2) at graphicsitems/qdeclarativevisualitemmodel.cpp:1312
 #6  0xf69f7519 in QDeclarativeVisualDataModel::qt_metacall (this=0x853bf30, _c=QMetaObject::InvokeMetaMethod, _id=8, _a=0xffffc710)
     at .moc/debug-shared/moc_qdeclarativevisualitemmodel_p.cpp:398
 #7  0xf5a819ec in QMetaObject::metacall (object=0x853bf30, cl=QMetaObject::InvokeMetaMethod, idx=20, argv=0xffffc710) at kernel/qmetaobject.cpp:237
 #8  0xf5a950cf in QMetaObject::activate (sender=0x87ceba8, m=0x8227188, local_signal_index=7, argv=0xffffc710) at kernel/qobject.cpp:3280
 #9  0xf5aec638 in QAbstractItemModel::rowsRemoved (this=0x87ceba8, _t1=..., _t2=0, _t3=1) at .moc/debug-shared/moc_qabstractitemmodel.cpp:195
 #10 0xf5a7421d in QAbstractItemModel::endRemoveRows (this=0x87ceba8) at kernel/qabstractitemmodel.cpp:2466
 #11 0x081044af in Imap::Mailbox::SubtreeModel::handleRowsRemoved (this=0x87ceba8, parent=..., first=0, last=1) at /home/harmattan/trojita/src/Imap/Model/SubtreeModel.cpp:282
 #12 0x0813d1bb in Imap::Mailbox::SubtreeModel::qt_metacall (this=0x87ceba8, _c=QMetaObject::InvokeMetaMethod, _id=5, _a=0xffffc920)
     at /home/harmattan/trojita/build/moc_SubtreeModel.cpp:113
 #13 0x0813d518 in Imap::Mailbox::SubtreeModelOfMailboxModel::qt_metacall (this=0x87ceba8, _c=QMetaObject::InvokeMetaMethod, _id=30, _a=0xffffc920)
     at /home/harmattan/trojita/build/moc_SubtreeModel.cpp:252
 #14 0xf5a819ec in QMetaObject::metacall (object=0x87ceba8, cl=QMetaObject::InvokeMetaMethod, idx=30, argv=0xffffc920) at kernel/qmetaobject.cpp:237
 #15 0xf5a950cf in QMetaObject::activate (sender=0x88ad108, m=0x8227188, local_signal_index=7, argv=0xffffc920) at kernel/qobject.cpp:3280
 #16 0xf5aec638 in QAbstractItemModel::rowsRemoved (this=0x88ad108, _t1=..., _t2=0, _t3=1) at .moc/debug-shared/moc_qabstractitemmodel.cpp:195
 #17 0xf5a7421d in QAbstractItemModel::endRemoveRows (this=0x88ad108) at kernel/qabstractitemmodel.cpp:2466
 #18 0x080c8725 in Imap::Mailbox::MailboxModel::handleRowsRemoved (this=0x88ad108, parent=..., first=1, last=2) at /home/harmattan/trojita/src/Imap/Model/MailboxModel.cpp:325
 #19 0x0813afa7 in Imap::Mailbox::MailboxModel::qt_metacall (this=0x88ad108, _c=QMetaObject::InvokeMetaMethod, _id=5, _a=0xffffcad0)
     at /home/harmattan/trojita/build/moc_MailboxModel.cpp:93
 #20 0xf5a819ec in QMetaObject::metacall (object=0x88ad108, cl=QMetaObject::InvokeMetaMethod, idx=30, argv=0xffffcad0) at kernel/qmetaobject.cpp:237
 #21 0xf5a950cf in QMetaObject::activate (sender=0x87c88c0, m=0x8227188, local_signal_index=7, argv=0xffffcad0) at kernel/qobject.cpp:3280
 #22 0xf5aec638 in QAbstractItemModel::rowsRemoved (this=0x87c88c0, _t1=..., _t2=1, _t3=2) at .moc/debug-shared/moc_qabstractitemmodel.cpp:195
 #23 0xf5a7421d in QAbstractItemModel::endRemoveRows (this=0x87c88c0) at kernel/qabstractitemmodel.cpp:2466
 #24 0x080df418 in Imap::Mailbox::Model::replaceChildMailboxes (this=0x87c88c0, mailboxPtr=0x8c3ac98, mailboxes=...) at /home/harmattan/trojita/src/Imap/Model/Model.cpp:496
 #25 0x080dec3b in Imap::Mailbox::Model::finalizeList (this=0x87c88c0, parser=0x88f1620, mailboxPtr=0x8c3ac98) at /home/harmattan/trojita/src/Imap/Model/Model.cpp:402
 #26 0x08126910 in Imap::Mailbox::ListChildMailboxesTask::handleStateHelper (this=0x8b33f30, resp=0x8cd0580)
     at /home/harmattan/trojita/src/Imap/Tasks/ListChildMailboxesTask.cpp:96
 #27 0x0811d1a2 in Imap::Mailbox::ImapTask::handleState (this=0x8b33f30, resp=0x8cd0580) at /home/harmattan/trojita/src/Imap/Tasks/ImapTask.cpp:106
 #28 0x08179c00 in Imap::Responses::State::plug (this=0x8cd0580, task=0x8b33f30) at /home/harmattan/trojita/src/Imap/Parser/Response.cpp:1612
 #29 0x080dda70 in Imap::Mailbox::Model::responseReceived (this=0x87c88c0, it=...) at /home/harmattan/trojita/src/Imap/Model/Model.cpp:232
 #30 0x080dd663 in Imap::Mailbox::Model::responseReceived (this=0x87c88c0, parser=0x88f1620) at /home/harmattan/trojita/src/Imap/Model/Model.cpp:173
 #31 0x0813b659 in Imap::Mailbox::Model::qt_metacall (this=0x87c88c0, _c=QMetaObject::InvokeMetaMethod, _id=38, _a=0x8bee9b0) at /home/harmattan/trojita/build/moc_Model.cpp:210
 #32 0xf5a819ec in QMetaObject::metacall (object=0x87c88c0, cl=QMetaObject::InvokeMetaMethod, idx=62, argv=0x8bee9b0) at kernel/qmetaobject.cpp:237
 #33 0xf5a8cdf9 in QMetaCallEvent::placeMetaCall (this=0x8789c88, object=0x87c88c0) at kernel/qobject.cpp:537
 #34 0xf5a8d400 in QObject::event (this=0x87c88c0, e=0x1) at kernel/qobject.cpp:1219
 #35 0xf5cecd49 in QApplicationPrivate::notify_helper (this=0x822d290, receiver=0x87c88c0, e=0x8789c88) at kernel/qapplication.cpp:4478
 #36 0xf5cf8a93 in QApplication::notify (this=0xffffd590, receiver=0x87c88c0, e=0x8789c88) at kernel/qapplication.cpp:4357
 #37 0xf5a797c8 in QCoreApplication::notifyInternal (this=0xffffd590, receiver=0x87c88c0, event=0x8789c88) at kernel/qcoreapplication.cpp:787
 #38 0xf5a7bfa4 in sendEvent (receiver=0x0, event_type=0, data=0x822d588) at kernel/qcoreapplication.h:215
 #39 QCoreApplicationPrivate::sendPostedEvents (receiver=0x0, event_type=0, data=0x822d588) at kernel/qcoreapplication.cpp:1428
 #40 0xf5a7c267 in QCoreApplication::sendPostedEvents (receiver=0x0, event_type=0) at kernel/qcoreapplication.cpp:1321
 #41 0xf5aac0e6 in sendPostedEvents (s=0x822fb18) at kernel/qcoreapplication.h:220
 #42 postEventSourceDispatch (s=0x822fb18) at kernel/qeventdispatcher_glib.cpp:277
 #43 0xf45a792b in g_main_context_dispatch () from /lib/libglib-2.0.so.0
 #44 0xf45abfb9 in ?? () from /lib/libglib-2.0.so.0
 #45 0xf45ac139 in g_main_context_iteration () from /lib/libglib-2.0.so.0
 #46 0xf5aabc4e in QEventDispatcherGlib::processEvents (this=0x822d888, flags=...) at kernel/qeventdispatcher_glib.cpp:422
 #47 0xf5db392b in QGuiEventDispatcherGlib::processEvents (this=0x822d888, flags=...) at kernel/qguieventdispatcher_glib.cpp:204
 #48 0xf5a782f1 in QEventLoop::processEvents (this=0xffffd4e4, flags=...) at kernel/qeventloop.cpp:149
 #49 0xf5a78768 in QEventLoop::exec (this=0xffffd4e4, flags=...) at kernel/qeventloop.cpp:201
 #50 0xf5a7c311 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1064
 #51 0xf5ceb979 in QApplication::exec () at kernel/qapplication.cpp:3752
 #52 0x080bcd33 in main (argc=1, argv=0xffffd684) at /home/harmattan/trojita/src/Harmattan/main.cpp:47