backend-drm: check that outputs are in fact ours

This is another followup to ffc011d6a3
("backend-drm: check that outputs and heads are in fact ours") which missed
some places.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>

libweston/backend-drm/kms.c

@@ -1103,11 +1103,13 @@ drm_pending_state_apply_atomic(struct drm_pending_state *pending_state,
 		wl_list_for_each(head_base,
 				 &b->compositor->head_list, compositor_link) {
 			struct drm_property_info *info;
+			head = to_drm_head(head_base);
+			if (!head)
+				continue;
 
 			if (weston_head_is_enabled(head_base))
 				continue;
 
-			head = to_drm_head(head_base);
 			connector_id = head->connector.connector_id;
 			if (head->connector.device != device)
 				continue;