Fix crash when authentication fails. Fix #7913

- Fix pointer dereferencing
- Remove call to uninitialized button
As stated by Valgrind:

==13851== Use of uninitialised value of size 8
==13851==    at 0x9961780: QWidget::setEnabled(bool) (in /usr/lib/x86_64-linux-gnu/libQtGui.so.4.8.2)
==13851==    by 0x27600FB1: QgsWFSSourceSelect::capabilitiesReplyFinished() (qgswfssourceselect.cpp:211)
==13851==    by 0x2760B78F: QgsWFSSourceSelect::qt_static_metacall(QObject*, QMetaObject::Call, int, void**) (moc_qgswfssourceselect.cxx:156)
==13851==    by 0x93F554E: QMetaObject::activate(QObject*, QMetaObject const*, int, void**) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.2)
==13851==    by 0x2760A458: QgsWFSCapabilities::gotCapabilities() (moc_qgswfscapabilities.cxx:103)
==13851==    by 0x275F88CD: QgsWFSCapabilities::capabilitiesReplyFinished() (qgswfscapabilities.cpp:156)
==13851==    by 0x2760A331: QgsWFSCapabilities::qt_static_metacall(QObject*, QMetaObject::Call, int, void**) (moc_qgswfscapabilities.cxx:54)
==13851==    by 0x93F554E: QMetaObject::activate(QObject*, QMetaObject const*, int, void**) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.2)
==13851==    by 0xA46E824: ??? (in /usr/lib/x86_64-linux-gnu/libQtNetwork.so.4.8.2)
==13851==    by 0xA4DC904: ??? (in /usr/lib/x86_64-linux-gnu/libQtNetwork.so.4.8.2)
==13851==    by 0x93F4A2D: QObject::event(QEvent*) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.2)
==13851==    by 0x991470B: QApplicationPrivate::notify_helper(QObject*, QEvent*) (in /usr/lib/x86_64-linux-gnu/libQtGui.so.4.8.2)
==13851==
GRASS shell cursor shift fix better
Merge pull request #623 from biolds/master

Fix crash when authentication fails. Fix #7913
Add GetContext request to QGIS WMS Server

This non standard request retruns the QGIS project as an OWSContext
document version 0.3.1.
This request gives more interoperability to QGIS Server that can be
directly used in client supporting OWSContext as config file.
This function has been funded by Ifremer.