net: qualcomm: rmnet: move null check on dev before dereferecing it

Currently dev is dereferenced by the call dev_net(dev) before dev is null
checked.  Fix this by null checking dev before the potential null
pointer dereference.

Detected by CoverityScan, CID#1462955 ("Dereference before null check")

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

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

@@ -301,10 +301,13 @@ static int rmnet_changelink(struct net_device *dev, struct nlattr *tb[],
 	struct rmnet_port *port;
 	u16 mux_id;
 
+	if (!dev)
+		return -ENODEV;
+
 	real_dev = __dev_get_by_index(dev_net(dev),
 				      nla_get_u32(tb[IFLA_LINK]));
 
-	if (!real_dev || !dev || !rmnet_is_real_dev_registered(real_dev))
+	if (!real_dev || !rmnet_is_real_dev_registered(real_dev))
 		return -ENODEV;
 
 	port = rmnet_get_port_rtnl(real_dev);