disp: msm: dp: check branch device when adding mst sim port

When adding a new MST sim port check if the branch device was
destroyed already to avoid dereferencing a null pointer.

Change-Id: I8e6f82c5a6772a9b85d24feca52368da2027e96d
Signed-off-by: Rajkumar Subbiah <rsubbia@codeaurora.org>

msm/dp/dp_mst_drm.c

@@ -267,6 +267,10 @@ static void dp_mst_sim_add_port(struct dp_mst_private *mst,
 	struct drm_dp_mst_port *port;
 
 	mstb = mst->mst_mgr.mst_primary;
+	if (!mstb) {
+		DP_ERR("Unable to add port. mst branch device was destroyed\n");
+		return;
+	}
 
 	port = kzalloc(sizeof(*port), GFP_KERNEL);
 	if (!port)