Fix crash (segfault) in node editor.

In some cases, we can have nodegroups without attached nodetree, this case is checked
in top `node_group_has_output()` entry point, but not in recursive `node_group_has_output_dfs()`.

Issue appeared after rB42824f4403db3a35, presumably because check on node flag itself
was skipping that case, which is no more possible when using ID tags.

Reported by sebastian_k over IRC, thanks.

source/blender/editors/space_node/node_relationships.c

@@ -107,7 +107,7 @@ static bool node_group_has_output_dfs(bNode *node)
 	     current_node = current_node->next)
 	{
 		if (current_node->type == NODE_GROUP) {
-			if (node_group_has_output_dfs(current_node)) {
+			if (current_node->id && node_group_has_output_dfs(current_node)) {
 				return true;
 			}
 		}