rmnet_core: Check if port is valid before tx

Add check to ensure that rmnet_port value returned
by rmnet_get_port is not null before we go ahead
and try to queue up an skb to be transmitted via it.

Change-Id: I7890f4f2bcbb8dd300957c4fb12ec77f0412f4a6
Signed-off-by: Conner Huff <chuff@codeaurora.org>

core/rmnet_map_data.c

@@ -1641,6 +1641,10 @@ void rmnet_map_tx_qmap_cmd(struct sk_buff *qmap_skb, u8 ch, bool flush)
 		ch = RMNET_DEFAULT_AGG_STATE;
 
 	port = rmnet_get_port(qmap_skb->dev);
+	if (!port) {
+		kfree_skb(qmap_skb);
+		return;
+	}
 	state = &port->agg_state[ch];
 
 	if (!flush)