Don't crash in wxGUIEventLoop::Exit() if not running in wxX11.

Don't crash in wxGUIEventLoop::Exit() if not running in wxX11.

The implementation of wxEventLoop::IsRunning() has changed since this code was
written and it doesn't check for m_impl != NULL any more. Because of this,
calling Exit() for an active but not running event loop resulted in a crash in
wxX11.

Fix this by doing nothing in this case. This seems better than asserting as
the event handling code exits the loop if an event handler throws an exception
and the loop might not be running in this case yet (events could be processed
because of a wxYield() call).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66098 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775