Fix command timeout handling when there's no stream

The media stream could be gone by the time that the AVDTP command timeout
expires (e.g. if the media transport channel gets closed prematurely). In
such a case there's no need to send a separate ABORT command but we can
directly proceed with closing the signalling channel.
diff --git a/audio/avdtp.c b/audio/avdtp.c
index a2f0aaa..41ded83 100644
--- a/audio/avdtp.c
+++ b/audio/avdtp.c
@@ -2290,6 +2290,9 @@
 		goto failed;
 	}
 
+	if (!stream)
+		goto failed;
+
 	memset(&sreq, 0, sizeof(sreq));
 	sreq.acp_seid = seid;