Work around the problem with not exiting event loop under MSW.

Work around the problem with not exiting event loop under MSW.

Changes of r71304 broke the event loop termination under MSW, limit them to
only the case in which they were really needed, i.e. when loop is really
exited by a pending event handler.

This is only a temporary solution, we need something better in longer term.

See #14250.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71329 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775