Fix stream handling when the media transport disconnects prematurely

We might not have a stream anymore when receiving the reply to a AVDTP
command that assumes that there is an existing stream. So ignore the
replies to these commands if we don't have a stream anymore.
diff --git a/audio/avdtp.c b/audio/avdtp.c
index 2a27ac8..efc001d 100644
--- a/audio/avdtp.c
+++ b/audio/avdtp.c
@@ -2580,6 +2580,16 @@
 		if (!(next && next->signal_id == AVDTP_GET_CAPABILITIES))
 			finalize_discovery(session, 0);
 		return TRUE;
+	}
+
+	/* The remaining commands require an existing stream so bail out
+	 * here if the stream got unexpectedly disconnected */
+	if (!stream) {
+		debug("AVDTP: stream was closed while waiting for reply");
+		return TRUE;
+	}
+
+	switch (signal_id) {
 	case AVDTP_SET_CONFIGURATION:
 		debug("SET_CONFIGURATION request succeeded");
 		return avdtp_set_configuration_resp(session, stream,