Update OALAudioTrack.m

Changed because it caused a crash in Apportable. Worked fine under iOS.

Explanation:
Apple's docs say that we can access 'error' parameter only if the value returned by [AVAudioPlayer initWithContentsOfURL:error:] is nil.
Update OALAudioTrack.m
Merge pull request #63 from jacopious/patch-1

Update OALAudioTrack.m