wlr_output_layout_contains_point: handle outputs that aren't in the layout

Instead of crashing, return `false` when the specified output isn't part
of the layout, as we can be sure that it doesn't contain the specified
point.

types/wlr_output_layout.c

@@ -231,6 +231,9 @@ bool wlr_output_layout_contains_point(struct wlr_output_layout *layout,
 	if (reference) {
 		struct wlr_output_layout_output *l_output =
 			wlr_output_layout_get(layout, reference);
+		if (!l_output) {
+			return false;
+		}
 		struct wlr_box output_box;
 		output_layout_output_get_box(l_output, &output_box);
 		return wlr_box_contains_point(&output_box, lx, ly);