Fix crash when accessing clipboard before entering the main loop.

Fix crash when accessing clipboard before entering the main loop.

Check that we have an event loop running before yielding to it.

Notice that we really do need an event loop in wxClipboardSync as events must
be dispatched for the clipboard code to work so while not using the event loop
if it doesn't exist would have fixed a crash as well, it wouldn't allow to use
the clipboard in e.g. wxApp-derived class OnInit().

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63599 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775