vo: unref waiting image before destroying VO driver

This is in theory more correct with respect to hardware decoding. With
hardware decoding, the VOs play the role of the video surface allocator,
and nothing is allowed to reference surfaces past the VO lifetime. But
in theory waiting_mpi could be a reference to a HW video surface, so it
should be relased before the VO is uninitialized.

video/out/vo.c

@@ -289,8 +289,8 @@ void vo_destroy(struct vo *vo)
 {
     if (vo->registered_fd != -1)
         mp_input_rm_key_fd(vo->input_ctx, vo->registered_fd);
+    mp_image_unrefp(&vo->waiting_mpi);
     vo->driver->uninit(vo);
-    talloc_free(vo->waiting_mpi);
     talloc_free(vo);
 }