Avoid sending spurious socket read notifications in wxMSW.

Avoid sending spurious socket read notifications in wxMSW.

If a read notification is generated for a socket, it should be possible to
read something from it without blocking but this doesn't seem to be always the
case under MSW for some reason. And this results in all sorts of problems in
wxSocket and wxIPC code, so check for this at wxSocketImpl level and not send
the notification at all if there is no data to read.

See #11528.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64565 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775