Fix GC problem with vm_handles in thread

runtime/vm/thread_registry.h

Index: runtime/vm/thread_registry.h
diff --git a/runtime/vm/thread_registry.h b/runtime/vm/thread_registry.h
index daa3cdd2b8d3f7314fc855958c86cf6d50cfd2ba..90d83f0b67876ca1739fbb5d66e0ecab0e08aadb 100644
--- a/runtime/vm/thread_registry.h
+++ b/runtime/vm/thread_registry.h
@@ -129,8 +129,10 @@ class ThreadRegistry {
       if (state.zone != NULL) {
         state.zone->VisitObjectPointers(visitor);
       }
-      ASSERT(entry.thread != NULL);
-      entry.thread->VisitObjectPointers(visitor);
+      if (entry.scheduled) {
+        ASSERT(entry.thread != NULL);
+        entry.thread->VisitObjectPointers(visitor);
+      }
       // Iterate over all the stack frames and visit objects on the stack.
       StackFrameIterator frames_iterator(state.top_exit_frame_info,
                                          validate_frames);