ProxyInputStream::Read() assigns the `offset` field, which is the
wrong offset because it does not consider Icy metadata removed from
the stream.  Therefore, after every ProxyInputStream::Read() call,
IcyInputStream::Read() needs to override this offset.  This was
missing at the end of the stream, when Read()==0.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1216