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==