use the currently active event loop for the event dispatching instead of wxYield...

use the currently active event loop for the event dispatching instead of wxYield(), this allows to create local event loops when using non-blocking sockets; assert if there is no active event loop under MSW as sockets won't work without it

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@56257 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775