osxvideosink: fix deadlock on gst_element_set_state(NULL)
The gst_element_set_state(NULL)
call finally lead to gst_osx_video_sink_osxwindow_destroy
which will be getting blocked on gst_osx_video_sink_call_from_main_thread
because (void) showFrame: (GstBufferObject *) object
is never finished trying to get the same lock which gst_osx_video_sink_osxwindow_destroy
already keep