net: qualcomm: rmnet: check for null ep to avoid null pointer dereference

The call to rmnet_get_endpoint can potentially return NULL so check
for this to avoid any subsequent null pointer dereferences on a NULL
ep.

Detected by CoverityScan, CID#1465385 ("Dereference null return value")

Fixes: 23790ef1 ("net: qualcomm: rmnet: Allow to configure flags for existing devices")
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/qualcomm/rmnet/rmnet_config.c

@@ -312,6 +312,8 @@ static int rmnet_changelink(struct net_device *dev, struct nlattr *tb[],
 	if (data[IFLA_RMNET_MUX_ID]) {
 		mux_id = nla_get_u16(data[IFLA_RMNET_MUX_ID]);
 		ep = rmnet_get_endpoint(port, priv->mux_id);
+		if (!ep)
+			return -ENODEV;
 
 		hlist_del_init_rcu(&ep->hlnode);
 		hlist_add_head_rcu(&ep->hlnode, &port->muxed_ep[mux_id]);