Fix #133786: Crash when modifying a Grease Pencil NLA track

The issue is that `ANIM_animdata_filter` with the flag `ANIMFILTER_ANIMDATA`
did not actually return only animdata.

The reason for this is in `animdata_filter_grease_pencil_data`,
where there is special handling for this flag.

It seems that returning non-animdata things is expected for Grease Pencil.
So my solution is to just verify the type of `bAnimListElem` before use.

Pull Request: https://projects.blender.org/blender/blender/pulls/133853

source/blender/editors/space_nla/nla_edit.cc

@@ -69,6 +69,9 @@ void ED_nla_postop_refresh(bAnimContext *ac)
     if (!ale->adt) {
       continue;
     }
+    if (ale->type != ANIMTYPE_ANIMDATA) {
+      continue;
+    }
     /* performing auto-blending, extend-mode validation, etc. */
     BKE_nla_validate_state(static_cast<AnimData *>(ale->data));