Fix mem leak found by asan

Indirect leak of 48 byte(s) in 1 object(s) allocated from:

#0 0x7f4fa41e3900 in operator new(unsigned long)

(/usr/lib64/libasan.so.5+0xed900)

#1 0x7f4f96a455b2 in Okular::Annotation::Window::Window()

/compile/kde5/framework/kde/kdegraphics/okular/core/annotations.cpp:341

#2 0x7f4f96a46207 in Okular::AnnotationPrivate::AnnotationPrivate()

/compile/kde5/framework/kde/kdegraphics/okular/core/annotations.cpp:493

#3 0x7f4f96a7f605 in

Okular::TextAnnotationPrivate::TextAnnotationPrivate()
(libOkular5Core.so.9+0xb9605)

#4 0x7f4f96a57ceb in Okular::TextAnnotation::TextAnnotation()

/compile/kde5/framework/kde/kdegraphics/okular/core/annotations.cpp:1042

#5 0x7f4f96e3e04f in EditAnnotToolDialog::createStubAnnotation()

/compile/kde5/framework/kde/kdegraphics/okular/conf/editannottooldialog.cpp:300

#6 0x7f4f96e2aa8b in

EditAnnotToolDialog::EditAnnotToolDialog(QWidget*, QDomElement const&)
/compile/kde5/framework/kde/kdegraphics/okular/conf/editannottooldialog.cpp:97

#7 0x7f4f96e52bbb in WidgetAnnotTools::slotAdd()

/compile/kde5/framework/kde/kdegraphics/okular/conf/widgetannottools.cpp:151

#8 0x7f4f96e5799c in

QtPrivate::FunctorCall<QtPrivate::IndexesList<>, QtPrivate::List<>,
void, void (WidgetConfigurationToolsBase::*)()>::call(void
(WidgetConfigurationToolsBase::*)(),