Fix join error by caching vTable in mapReduceTriplets

graph/src/main/scala/org/apache/spark/graph/impl/GraphImpl.scala

@@ -258,6 +258,9 @@ class GraphImpl[VD: ClassManifest, ED: ClassManifest] protected (
       }
     }
 
+    // Permit joining the result of mapReduceTriplets back with vTable
+    vTable.cache()
+
     // do the final reduction reusing the index map
     vTable.aggregateUsingIndex(preAgg, reduceFunc)
   } // end of mapReduceTriplets