SPARK-2553. Fix compile error

Author: Sandy Ryza <sandy@cloudera.com>

Closes #1479 from sryza/sandy-spark-2553 and squashes the following commits:

2cb5ed8 [Sandy Ryza] SPARK-2553. Fix compile error

core/src/main/scala/org/apache/spark/rdd/CoGroupedRDD.scala

@@ -185,6 +185,7 @@ class CoGroupedRDD[K](@transient var rdds: Seq[RDD[_ <: Product2[K, _]]], part:
           combiner1(depNum) ++= combiner2(depNum)
           depNum += 1
         }
+        combiner1
       }
     new ExternalAppendOnlyMap[K, CoGroupValue, CoGroupCombiner](
       createCombiner, mergeValue, mergeCombiners)