Adds a null check in NuPlayer::closeAudioSink.

This was causing a null deref in cts/security/gce-all.

Bug: 142652210
Test: Run cts/security/gce-all, observe no more crash.
Change-Id: If2003b650f69e88825219113ce1b7d70edc4731d

media/libmediaplayerservice/nuplayer/NuPlayer.cpp

@@ -1798,7 +1798,9 @@ void NuPlayer::tryOpenAudioSinkForOffload(
 }
 
 void NuPlayer::closeAudioSink() {
-    mRenderer->closeAudioSink();
+    if (mRenderer != NULL) {
+        mRenderer->closeAudioSink();
+    }
 }
 
 void NuPlayer::restartAudio(