Merge \"Add null check for rtsp source\" into nyc-mr1-dev

    size_t numTracks = mTracks.size();

    size_t preparedCount, underflowCount, overflowCount, startCount;

    preparedCount = underflowCount = overflowCount = startCount = 0;

    for (size_t i = 0; i < numTracks; ++i) {

    size_t count = numTracks;

    for (size_t i = 0; i < count; ++i) {

        status_t finalResult;

        TrackInfo *info = &mTracks.editItemAt(i);

        sp<AnotherPacketSource> src = info->mSource;

        if (src == NULL) {

            --numTracks;

            continue;

        }

        int64_t bufferedDurationUs = src->getBufferedDurationUs(&finalResult);

        // isFinished when duration is 0 checks for EOS result only