MediaPlayerService: allow next player to be NULL

Bug: 31155917
Bug: 30204103
Change-Id: I9a2a59ddb900fc942e7c19b31b53a110d790474c
diff --git a/media/libmediaplayerservice/MediaPlayerService.cpp b/media/libmediaplayerservice/MediaPlayerService.cpp
index c5a6e49..bd16e91 100644
--- a/media/libmediaplayerservice/MediaPlayerService.cpp
+++ b/media/libmediaplayerservice/MediaPlayerService.cpp
@@ -1092,7 +1092,7 @@
     ALOGV("setNextPlayer");
     Mutex::Autolock l(mLock);
     sp<Client> c = static_cast<Client*>(player.get());
-    if (!mService->hasClient(c)) {
+    if (c != NULL && !mService->hasClient(c)) {
       return BAD_VALUE;
     }