Fix sending incorrect AVDTP commands after a timeout occurs

Fix sending incorrect AVDTP commands after a timeout occurs

Since request_timeout() does callbacks into a2dp.c before sending the
ABORT command we need to make sure that none of the callbacks can
trigger sending any further AVDTP commands. This is easiest done by
setting the stream->abort_int flag early in the request_timeout function
and then checking for it in the send_request function.