Return exit code as signed integer from wxExecute(wxEXEC_SYNC).

Return exit code as signed integer from wxExecute(wxEXEC_SYNC).

The caller expects the function to return -1 if the child process exited with
-1 error code and not 255 that was returned before. The function is also
documented as returning -1 if the execution fails which wasn't true neither.

Fix this by explicitly handling the exit code as signed.

Closes #11824.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63710 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775