paplayer handle prepare stream failure.

I've encountered a crash in gdb after  log message:
```
PAPlayer::PrepareStream - Failed to get IAEStream 
```

what cause crash is at line in PAPlayer::ProcessStream with si->m_stream == NULL:
```
    si->m_stream->RegisterAudioCallback(m_audioCallback);
```

I checked only place called PrepareStream is in PAPlayer::QueueNextFileEx where did not check failure, so here is the fix for it, please check whether it is correct..