surface: ensure buffer is reset to NULL in surface_state_move

When surface_state_move processes a NULL commit, make sure to
reset state->buffer to NULL instead of leaving behind an old
wlr_buffer.

types/wlr_surface.c

@@ -284,8 +284,9 @@ static void surface_state_move(struct wlr_surface_state *state,
 		surface_state_set_buffer(state, next->buffer_resource);
 		surface_state_reset_buffer(next);
 
-		if (next->buffer) {
 		wlr_buffer_unlock(state->buffer);
+		state->buffer = NULL;
+		if (next->buffer) {
 			state->buffer = wlr_buffer_lock(next->buffer);
 		}
 		wlr_buffer_unlock(next->buffer);