Fix segfault in view_unmap()

If the last remaining view on a workspace is unmapped and the workspace
is not visible, parent will be a C_OUTPUT. Call the arrange_output()
function in this case.

sway/tree/view.c

@@ -302,7 +302,11 @@ void view_unmap(struct sway_view *view) {
 	view->swayc = NULL;
 	view->surface = NULL;
 
+	if (parent->type == C_OUTPUT) {
+		arrange_output(parent);
+	} else {
 		arrange_children_of(parent);
+	}
 }
 
 void view_update_position(struct sway_view *view, double ox, double oy) {