Blender Git Loki

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.