6742641: G1: NullPointerException during GCOld

Summary: An update buffer is not processed correctly, which causes roots into the collection set not to be scanned and, hence, for the heap to be corrupted. The cause is that an object is accessed after it has been explicitly deleted, which causes a race.
Reviewed-by: jcoomes, ysr

src/share/vm/gc_implementation/g1/dirtyCardQueue.cpp

@@ -208,12 +208,13 @@ apply_closure_to_completed_buffer_helper(int worker_i,
                                               nd->index, _sz,
                                               true, worker_i);
     void** buf = nd->buf;
+    size_t index = nd->index;
     delete nd;
     if (b) {
       deallocate_buffer(buf);
       return true;  // In normal case, go on to next buffer.
     } else {
-      enqueue_complete_buffer(buf, nd->index, true);
+      enqueue_complete_buffer(buf, index, true);
       return false;
     }
   } else {