tdf#118573: Acquire solar mutex when calling Application::Reschedule

Change-Id: I252973ad5902738c2bb684f6bb70bc4c46500bae
Reviewed-on: https://gerrit.libreoffice.org/62873Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>

vcl/win/window/salframe.cxx

@@ -5686,7 +5686,10 @@ static LRESULT CALLBACK SalFrameWndProc( HWND hWnd, UINT nMsg, WPARAM wParam, LP
                 // messages in the message queue and dispatch them before we return control to the system.
 
                 if ( nRet )
+                {
+                    SolarMutexGuard aGuard;
                     while ( Application::Reschedule( true ) );
+                }
             }
             else
             {