Special return data mark not needed on gen2 frames

It is only relevant for those on the call stack, and so already gone
by the time something is marked as a gen2 only frame.

src/gc/roots.c

@@ -310,10 +310,6 @@ void MVM_gc_root_add_frame_roots_to_worklist(MVMThreadContext *tc, MVMGCWorklist
     if (cur_frame->context_object)
         MVM_gc_worklist_add(tc, worklist, &cur_frame->context_object);
 
-    /* Mark special return data, if needed. */
-    if (cur_frame->special_return_data && cur_frame->mark_special_return_data)
-        cur_frame->mark_special_return_data(tc, cur_frame, worklist);
-
     /* Mark any continuation tags. */
     if (cur_frame->continuation_tags) {
         MVMContinuationTag *tag = cur_frame->continuation_tags;
@@ -331,6 +327,10 @@ void MVM_gc_root_add_frame_roots_to_worklist(MVMThreadContext *tc, MVMGCWorklist
         /* Remember how many items were on the GC worklist before we began. */
         MVMuint32 orig_worklist_items = worklist->items;
 
+        /* Mark special return data, if needed. */
+        if (cur_frame->special_return_data && cur_frame->mark_special_return_data)
+            cur_frame->mark_special_return_data(tc, cur_frame, worklist);
+
         /* Mark any dynamic variable cache name; this one never levels beyond
          * frame->tc becoming NULL, so isn't actually barried anywhere. */
         if (cur_frame->dynlex_cache_name)