sc: fix crash in ScVbaEventListener::processWindowResizeEvent()

This was crashing in CppunitTest_sc_macros_test on Windows with
--enable-mergelibs (release build), because the first invocation of
processWindowResizeEvent() deleted the vcl::Window, and the
maControllers.count(pWindow) test creates a VclPtr for it, so ends up
with a double-free.

TODO: is processWindowResizeEvent() supposed to be idempotent?
It would be possible to detect that there is already an event posted by
checking m_PostedWindows in postWindowResizeEvent().

Change-Id: I7b72f2baf21bb8223e9fe4bd929d826217b920e5
(cherry picked from commit 520514cf)
Reviewed-on: https://gerrit.libreoffice.org/19022Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

sc/source/ui/vba/vbaeventshelper.cxx

@@ -169,7 +169,7 @@ private:
     uno::Reference< frame::XModel > mxModel;
     ScDocShell*         mpDocShell;
     WindowControllerMap maControllers;          /// Maps VCL top windows to their controllers.
-    std::set< VclPtr<vcl::Window> > maPostedWindows; /// Keeps processWindowResizeEvent windows from being deleted between postWindowResizeEvent and processWindowResizeEvent
+    std::multiset< VclPtr<vcl::Window> > m_PostedWindows; /// Keeps processWindowResizeEvent windows from being deleted between postWindowResizeEvent and processWindowResizeEvent
     VclPtr<vcl::Window>            mpActiveWindow; /// Currently activated window, to prevent multiple (de)activation.
     bool                mbWindowResized;        /// True = window resize system event processed.
     bool                mbBorderChanged;        /// True = borders changed system event processed.
@@ -471,7 +471,7 @@ void ScVbaEventListener::postWindowResizeEvent( vcl::Window* pWindow )
     {
         mbWindowResized = mbBorderChanged = false;
         acquire();  // ensure we don't get deleted before the timer fires
-        maPostedWindows.insert(pWindow);
+        m_PostedWindows.insert(pWindow);
         Application::PostUserEvent( LINK( this, ScVbaEventListener, processWindowResizeEvent ), pWindow );
     }
 }
@@ -504,7 +504,14 @@ IMPL_LINK( ScVbaEventListener, processWindowResizeEvent, vcl::Window*, pWindow )
             }
         }
     }
-    maPostedWindows.erase(pWindow);
+    {
+        // note: there may be multiple processWindowResizeEvent outstanding
+        // for pWindow, so it may have been added to m_PostedWindows multiple
+        // times - so this must delete exactly one of these elements!
+        auto const iter(m_PostedWindows.find(pWindow));
+        assert(iter != m_PostedWindows.end());
+        m_PostedWindows.erase(iter);
+    }
     release();
     return 0;
 }