GenericSource: reset mDrmManagerClient when mDataSource is cleared.

Bug: 25070434
Change-Id: Iade3472c496ac42456e42db35e402f7b66416f5b
(cherry picked from commit b41fd0d4929f0a89811bafcc4fd944b128f00ce2)
diff --git a/media/libmediaplayerservice/nuplayer/GenericSource.cpp b/media/libmediaplayerservice/nuplayer/GenericSource.cpp
index 82694bd..d62d0ae 100644
--- a/media/libmediaplayerservice/nuplayer/GenericSource.cpp
+++ b/media/libmediaplayerservice/nuplayer/GenericSource.cpp
@@ -481,6 +481,7 @@
             {
                 Mutex::Autolock _l(mDisconnectLock);
                 mDataSource.clear();
+                mDrmManagerClient = NULL;
                 mCachedSource.clear();
                 mHttpSource.clear();
             }