Fix MediaPlayerFlakyNetworkTest NPE

Bug 6288085

MediaPlayerFlakyNetworkTest sets mMediaPlayer to null so just check
whether its null before releasing.

Change-Id: I6fe00d7ccc155db98539aaf91a0f84da459b4e05
diff --git a/tests/tests/media/src/android/media/cts/MediaPlayerTestBase.java b/tests/tests/media/src/android/media/cts/MediaPlayerTestBase.java
index 0241357..ae85cd2 100644
--- a/tests/tests/media/src/android/media/cts/MediaPlayerTestBase.java
+++ b/tests/tests/media/src/android/media/cts/MediaPlayerTestBase.java
@@ -107,8 +107,12 @@
 
     @Override
     protected void tearDown() throws Exception {
-        mMediaPlayer.release();
-        mMediaPlayer2.release();
+        if (mMediaPlayer != null) {
+            mMediaPlayer.release();
+        }
+        if (mMediaPlayer2 != null) {
+            mMediaPlayer2.release();
+        }
         super.tearDown();
     }