Merge branch 'dev' of github.com:mesos/spark into dev

core/src/main/scala/spark/storage/BlockManager.scala

@@ -706,7 +706,9 @@ object BlockManager extends Logging {
   def dispose(buffer: ByteBuffer) {
     if (buffer != null && buffer.isInstanceOf[MappedByteBuffer]) {
       logDebug("Unmapping " + buffer)
-      buffer.asInstanceOf[DirectBuffer].cleaner().clean()
+      if (buffer.asInstanceOf[DirectBuffer].cleaner() != null) {
+        buffer.asInstanceOf[DirectBuffer].cleaner().clean()
+      }
     }
   }
 }