Issue 1710: Check if sound collection contains item before using it.

[Issue 1710](http://code.google.com/p/ankidroid/issues/detail?id=1710)

Sounds are played through in sequence on their own thread. Another thread could clear the collection of sounds, resulting in a crash after .get()ing a null object. Here, we check if the collection contains items before trying to use it, so the sequence terminates if the collection has been cleared elsewhere.