vcl: check mpWindowImpl for nullptr.

Crash in calc's mouse capture / end tracking.

Change-Id: If5b4cded8ebfc04d1fcf645a7b74184532ab2338
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/119034
Tested-by: Jenkins
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>

vcl/source/window/mouse.cxx

@@ -466,7 +466,8 @@ void Window::ReleaseMouse()
     {
         ImplSVData* pSVData = ImplGetSVData();
         pSVData->mpWinData->mpCaptureWin = nullptr;
-        mpWindowImpl->mpFrame->CaptureMouse( false );
+        if (mpWindowImpl && mpWindowImpl->mpFrame)
+            mpWindowImpl->mpFrame->CaptureMouse( false );
         ImplGenerateMouseMove();
     }
 }