Avoid overflowing the wake up when handling events in Unix console apps.

Avoid overflowing the wake up when handling events in Unix console apps.

Generating many wake ups from the worker threads could result in overflowing
the buffer of the pipe used to communicate with the main thread which, in
turn, resulted in other serious problems (deadlocks...).

Avoid this by only writing to the pipe if it is empty.

Closes #14166.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71089 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775