Fix valgrind warning

25334== Invalid read of size 1

25334== at 0xBBEA860: QGuiApplicationPrivate::mouseEventSource(QMouseEvent const*) (in /home/boud/dev/deps/lib/libQt5Gui.so.5.6.1)

25334== by 0x52B49F7: KisInputManager::Private::EventEater::eventFilter(QObject*, QEvent*) (kis_input_manager_p.cpp:83)

25334== by 0x52B3D81: KisInputManager::eventFilter(QObject*, QEvent*) (kis_input_manager.cpp:177)

25334== by 0xC5477B0: QCoreApplicationPrivate::sendThroughObjectEventFilters(QObject*, QEvent*) (in /home/boud/dev/deps/lib/libQt5Core.so.5.6.1)

25334== by 0xB3DCDC4: QApplicationPrivate::notify_helper(QObject*, QEvent*) (in /home/boud/dev/deps/lib/libQt5Widgets.so.5.6.1)

25334== by 0xB3E1449: QApplication::notify(QObject*, QEvent*) (in /home/boud/dev/deps/lib/libQt5Widgets.so.5.6.1)

25334== by 0x530D7B6: KisApplication::notify(QObject*, QEvent*) (KisApplication.cpp:511)

25334== by 0xC5478E4: QCoreApplication::notifyInternal2(QObject*, QEvent*) (in /home/boud/dev/deps/lib/libQt5Core.so.5.6.1)

25334== by 0xC5496C9: QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) (in /home/boud/dev/deps/lib/libQt5Core.so.5.6.1)

25334== by 0xC5986D2: ??? (in /home/boud/dev/deps/lib/libQt5Core.so.5.6.1)

25334== by 0x10F28C83: g_main_context_dispatch (in /usr/lib64/libglib-2.0.so.0.4400.1)

25334== by 0x10F28ED7: ??? (in /usr/lib64/libglib-2.0.so.0.4400.1)

25334== Address 0x1a45b759 is 7 bytes before a block of size 24 alloc'd

25334== at 0x4C29110: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)

25334== by 0xC3D3D78: QHashData::allocateNode(int) (in /home/boud/dev/deps/lib/libQt5Core.so.5.6.1)

25334== by 0xB41FFBB: QWidgetPrivate::init(QWidget*, QFlags<Qt::WindowType>) (in /home/boud/dev/deps/lib/libQt5Widgets.so.5.6.1)

25334== by 0xB4D3A43: QAbstractButton::QAbstractButton(QWidget*) (in /home/boud/dev/deps/lib/libQt5Widgets.so.5.6.1)

25334== by 0x6C8CFAD: KoDockWidgetTitleBarButton::KoDockWidgetTitleBarButton(QWidget*) (KoDockWidgetTitleBarButton.cpp:42)

25334== by 0x6C8C2CC: KoDockWidgetTitleBar::KoDockWidgetTitleBar(QDockWidget*) (KoDockWidgetTitleBar.cpp:67)

25334== by 0x533B57C: KisMainWindow::createDockWidget(KoDockFactoryBase*) (KisMainWindow.cpp:1859)

25334== by 0x53423A9: KisMainWindow::KisMainWindow() (KisMainWindow.cpp:312)

25334== by 0x534CF45: KisPart::createMainWindow() (KisPart.cpp:216)

25334== by 0x530FAB5: KisApplication::start(KisApplicationArguments const&) (KisApplication.cpp:387)

25334== by 0x4051DD: main (main.cc:246)