Fix crash in CMakePreferences::updateCache

Happened when invoking a solution involving starting up the
project config page.

Backtrace:

SUMMARY: AddressSanitizer: undefined-behavior
/home/kfunk/devel/src/kf5/kdevelop-stable/projectmanagers/cmake/settings/cmakepreferences.cpp:180:9
in

ASAN:DEADLYSIGNAL

31850==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000

(pc 0x7f10b452f8b7 bp 0x7fff26dce210 sp 0x7fff26dcdd20 T0)

#0 0x7f10b452f8b6 in CMakePreferences::updateCache(KDevelop::Path

const&)
/home/kfunk/devel/src/kf5/kdevelop-stable/projectmanagers/cmake/settings/cmakepreferences.cpp:180:9

#1 0x7f10b451a94f in CMakePreferences::buildDirChanged(int)

/home/kfunk/devel/src/kf5/kdevelop-stable/projectmanagers/cmake/settings/cmakepreferences.cpp:231:5

#2 0x7f10b454f049 in

QtPrivate::FunctorCall<QtPrivate::IndexesList<0>, QtPrivate::List<int>,
void, void (CMakePreferences::*)(int)>::call(void
(CMakePreferences::*)(int), CMakePreferences*, void**)
/usr/include/x86_64-linux-gnu/qt5/QtCore/qobjectdefs_impl.h:501:13

#3 0x7f10b454eb5d in void QtPrivate::FunctionPointer<void

(CMakePreferences::*)(int)>::call<QtPrivate::List<int>, void>(void
(CMakePreferences::*)(int), CMakePreferences*, void**)
/usr/include/x86_64-linux-gnu/qt5/QtCore/qobjectdefs_impl.h:520:13

#4 0x7f10b454e5ff in QtPrivate::QSlotObject<void

(CMakePreferences::*)(int), QtPrivate::List<int>, void>::impl(int,
QtPrivate::QSlotObjectBase*, QObject*, void**, bool*)
/usr/include/x86_64-linux-gnu/qt5/QtCore/qobject_impl.h:143:17

#5 0x7f10fb4b8e4e in QMetaObject::activate(QObject*, int, int,

void**) (/usr/lib/x86_64-linux-gnu/libQt5Core.so.5+0x2b4e4e)

#6 0x7f10fbe8f380 in QComboBox::currentIndexChanged(int)

(/usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5+0x26d380)

#7 0x7f10fbe917e0

(/usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5+0x26f7e0)

#8 0x7f10fbe93e0c

(/usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5+0x271e0c)

#9 0x7f10fbe9403e in QComboBox::setCurrentIndex(int)

(/usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5+0x27203e)

#10 0x7f10fbe99678

(/usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5+0x277678)

#11 0x7f10fb4b8fc9 in QMetaObject::activate(QObject*, int, int,

void**) (/usr/lib/x86_64-linux-gnu/libQt5Core.so.5+0x2b4fc9)

#12 0x7f10fb535323 in QAbstractItemModel::rowsInserted(QModelIndex

const&, int, int, QAbstractItemModel::QPrivateSignal)
(/usr/lib/x86_64-linux-gnu/libQt5Core.so.5+0x331323)

#13 0x7f10fb432775 in QAbstractItemModel::endInsertRows()

(/usr/lib/x86_64-linux-gnu/libQt5Core.so.5+0x22e775)

#14 0x7f10fbb38f32

(/usr/lib/x86_64-linux-gnu/libQt5Gui.so.5+0x45ef32)

#15 0x7f10fbe965bb in QComboBox::insertItems(int, QStringList

const&) (/usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5+0x2745bb)

#16 0x7f10b454148e in QComboBox::addItems(QStringList const&)

/usr/include/x86_64-linux-gnu/qt5/QtWidgets/qcombobox.h:174:11

#17 0x7f10b4526a23 in CMakePreferences::reset()

/home/kfunk/devel/src/kf5/kdevelop-stable/projectmanagers/cmake/settings/cmakepreferences.cpp:108:5

#18 0x7f10b4519e3d in

CMakePreferences::CMakePreferences(KDevelop::IPlugin*,
KDevelop::ProjectConfigOptions const&, QWidget*)
/home/kfunk/devel/src/kf5/kdevelop-stable/projectmanagers/cmake/settings/cmakepreferences.cpp:95:5

#19 0x7f10b4479bd7 in CMakeManager::perProjectConfigPage(int,

KDevelop::ProjectConfigOptions const&, QWidget*)
/home/kfunk/devel/src/kf5/kdevelop-stable/projectmanagers/cmake/cmakemanager.cpp:886:20

#20 0x7f11021ecfdc in

KDevelop::ProjectControllerPrivate::projectConfig(QObject*)
/home/kfunk/devel/src/kf5/kdevplatform-stable/shell/projectcontroller.cpp:143:36

#21 0x7f11021c380f in

KDevelop::ProjectController::configureProject(KDevelop::IProject*)
/home/kfunk/devel/src/kf5/kdevplatform-stable/shell/projectcontroller.cpp:976:5

#22 0x7f10afe80e42 in

DefinesAndIncludesManager::openConfigurationDialog(QString const&)
/home/kfunk/devel/src/kf5/kdevelop-stable/languages/plugins/custom-definesandincludes/definesandincludesmanager.cpp:224:9

...