Fix crash when disconnecting A2DP

When we disconnect an A2DP device by immediate power off and quickly
also initiate a disconnection from our side, a crash is occurring. This
is happening because of the disconnect_timeout coming after freeing the
session.
diff --git a/audio/avdtp.c b/audio/avdtp.c
index e6407bb..3493f61 100644
--- a/audio/avdtp.c
+++ b/audio/avdtp.c
@@ -685,7 +685,7 @@
 		remove_disconnect_timer(session);
 
 	if (session->device_disconnect) {
-		g_idle_add(disconnect_timeout, session);
+		session->dc_timer = g_idle_add(disconnect_timeout, session);
 		return;
 	}