raop: Clear packet buffer at the end of connection

When an RTSP connection is newly established, it's not only useless
but also harmful to keep the old retransmission packets. So cleanup
the buffer when the connection is torn down.
raop: Handle packet buffer count correctly

When searching the packet buffer, range check should be done
against the number of packets in the buffer, rather than the
buffer capacity.
Discard audio packets which can not be sent due to
blocking of the audio streaming port.