SFR 458544 Prevent an exception being thrown while stream is closed.
The RawBufferMux.RawBufferSourceStream effectively calls read() on itself
in its run() method (via handler.transferData()).  This throws an
exception if called when closed is true, so ensure we don't do that.

Hitting the exception means we sometimes (but not always) die - if there's
a hanging read() call coming in to the CachingPushBufferStream.  This can
happen if the call comes before the audio mixer realises the stream has
been torn down.