gdb/find_refs: Fix next_free_snap lookup

gdb/ghc_gdb/find_refs.py

@@ -223,7 +223,10 @@ def refs_dot(edges: List[Edge], roots: Set[Ptr], mut_list_reachables: Dict[int,
         seg_blkIdx = get_nonmoving_segment(ref.referring_field)
         if seg_blkIdx is not None:
             seg, blk = seg_blkIdx
-            snap = int(seg['next_free_snap'])
+            if 'next_free_snap' in seg.type.fields():
+                snap = int(seg['next_free_snap'])
+            else:
+                snap = int(gdb.parse_and_eval('Bdescr(%d)->nonmoving_segment.next_free_snap' % ref.referring_field.addr()))
             mark = seg['bitmap'][blk]
             if mark != 0:
                 color = 'blue'