Fix crash in lokit form control mouse handler

Fix crash in lokit form control mouse handler

Since 7adfecb0f5947ae258226c8d1652546f81577026

, clicking on form controls results in a crash like this one:

  0x00007f7c81092a7c in __pthread_kill_implementation () at /lib64/libc.so.6
  0x00007f7c81041226 in raise () at /lib64/libc.so.6
  0x00007f7c81029897 in abort () at /lib64/libc.so.6
  0x00007f7c812e4c60 in __gnu_debug::_Safe_sequence_base::_M_get_mutex() () at /lib64/libstdc++.so.6
  0x00007f7c2aaaf36f in std::_Optional_base_impl<PointerStyle, std::_Optional_base<PointerStyle, true, true> >::_M_get()
  (this=0x7f7c2c673b50 <LokControlHandler::postMouseEvent(SdrPage const*, SdrView const*, vcl::DocWindow&, int, Point, int, int, int)::eDocPointerStyle>)
  at /usr/include/c++/13/optional:477
  0x00007f7c2aaa9b24 in std::optional<PointerStyle>::operator*() &
  (this=0x7f7c2c673b50 <LokControlHandler::postMouseEvent(SdrPage const*, SdrView const*, vcl::DocWindow&, int, Point, int, int, int)::eDocPointerStyle>)
  at /usr/include/c++/13/optional:974
  0x00007f7c2aaa66bb in LokControlHandler::postMouseEvent(SdrPage const*, SdrView const*, vcl::DocWindow&, int, Point, int, int, int)
  (pPage=0x7f7c6ec35910, pDrawView=0x7f7c6f097a40, rMainWindow=..., nType=0, aPointHmm=Point = {...}, nCount=1, nButtons=1, nModifier=0)
  at core/include/sfx2/LokControlHandler.hxx:79
  0x00007f7c2aa9d784 in SwXTextDocument::postMouseEvent(int, int, int, int, int, int) (this=0x7f7c6eae94f0, nType=0, nX=1756, nY=4106, nCount=1, nButtons=1, nModifier=0)
  at core/sw/source/uibase/uno/unotxdoc.cxx:3741
  0x00007f7c7c8fb074 in doc_postMouseEvent(LibreOfficeKitDocument*, int, int, int, int, int, int) (pThis=0x7f7c6f37fe90, nType=0, nX=1756, nY=4106, nCount=1, nButtons=1, nModifier=0)
  at core/desktop/source/lib/init.cxx:5062
  0x00007f7c81642c9e in postMouseEventInThread(gpointer) (data=0x19aa340) at core/libreofficekit/source/gtk/lokdocview.cxx:2335
  0x00007f7c816441fb in lokThreadFunc(gpointer, gpointer) (data=0x19aa340) at core/libreofficekit/source/gtk/lokdocview.cxx:2567

Change-Id: I77e85df6a77adc033cc793488924ed676a9900be
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/155364
Tested-by: Jenkins
Reviewed-by: Thorsten Behrens <thorsten.behrens@allotropia.de>