Fix printing D-Bus errors when headset record could not be found

When connection is started via headset_config_stream there is no D-Bus
message to reply to.
diff --git a/audio/headset.c b/audio/headset.c
index 72bf5b6..53a594d 100644
--- a/audio/headset.c
+++ b/audio/headset.c
@@ -1445,7 +1445,8 @@
 		error("Unable to get service record: %s (%d)",
 							strerror(-err), -err);
 		p->err = -err;
-		error_connect_failed(dev->conn, p->msg, p->err);
+		if (p->msg)
+			error_connect_failed(dev->conn, p->msg, p->err);
 		goto failed;
 	}