batman-adv: don't pass a NULL hard_iface to batadv_hardif_put

In the case where hard_iface is NULL, the error path may pass a null
pointer to batadv_hardif_put causing a null pointer dereference error.
Avoid this by only calling the function if  hard_iface not null.

Detected by CoverityScan, CID#1466456 ("Explicit null dereferenced")

Fixes: 53dd9a68 ("batman-adv: add multicast flags netlink support")
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NSven Eckelmann <sven@narfation.org>
Signed-off-by: NSimon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/multicast.c

@@ -1536,7 +1536,7 @@ batadv_mcast_netlink_get_primary(struct netlink_callback *cb,
 
 	if (!ret && primary_if)
 		*primary_if = hard_iface;
-	else
+	else if (hard_iface)
 		batadv_hardif_put(hard_iface);
 
 	return ret;