Bug with clear next range

Created by: nweber

There appears to be a race condition that occurs sometimes in the BufferController where the controller thinks that it needs to prune some ranges, but it's not possible to do so. The BufferController already has checks to catch these scenarios, but the pruning state isn't flipped back to "not pruning", which causes the player to get stuck in a pruning state. This has the side effect of effectively halting playback since the ScheduleController will not executes requests while pruning.

This PR resets the pruning state if it is discovered that pruning cannot continue.