Fix a potential corruption in release_input_thread().

Fix a potential corruption in release_input_thread().

If the media instance owns the input and libvlc_media_instance_release()
is called immediately after libvlc_media_instance_stop(), there is a
chance that we try to free the input resources before the thread is
effectively terminated. To avoid this, we call input_DestroyThread()
instead of vlc_object_release() at the end of release_input_thread() when
the media instance owns the input thread.