[Audio] Spotify Enqueue cannot be stopped

# Other bugs

#### What were you trying to do?

Stop the bot from playing with `[p]stop` while a Spotify playlist was queuing.

#### What were you expecting to happen?

The bot to stop playing, the queue to clear, and the playlist to stop queuing.

#### What actually happened?

The current song is stopped and the queue is cleared, but the playlist continues adding to the queue and causes the next added song to play.

#### How can we reproduce this issue?

1. Start a slow-loading playlist, like a Spotify playlist.
2. Let it start loading until songs start playing.
3. Use `[p]stop` before the playlist finishes queuing.