Don't crash if the MediaCodecList is null.

We need to avoid calling the JavaMediaCodecListWrapper constructor if
the MediaCodecList is null because the constructor assumes its argument
to be non-null. Since throwing a JNI exception does not implicitly
return we need to return null to avoid entering the constructor.

Change-Id: If473d2f6f219719ff81c29307637f28b7e31af77

media/jni/android_media_MediaCodecList.cpp

@@ -105,6 +105,7 @@ static const JavaMediaCodecListWrapper *getCodecList(JNIEnv *env) {
             // This should never happen unless something is really wrong
             jniThrowException(
                         env, "java/lang/RuntimeException", "cannot get MediaCodecList");
+            return NULL;
         }
 
         sListWrapper.reset(new JavaMediaCodecListWrapper(mcl));