gnrc_netif: add missing gnrc_netif_release() to early return

The gnrc_netif_ipv6_addr_add_internal() function unconditionally
acquires the global netif rmutex lock but doesn't release this lock on
this specific path (i.e. if gnrc_netif_ipv6_group_join_internal failed).
This can cause a deadlock as no other thread will afterwards be capable
of acquiring the netif lock.

sys/net/gnrc/netif/gnrc_netif.c

@@ -561,6 +561,7 @@ int gnrc_netif_ipv6_addr_add_internal(gnrc_netif_t *netif,
               PRIkernel_pid "\n",
               ipv6_addr_to_str(addr_str, addr, sizeof(addr_str)),
               netif->pid);
+        gnrc_netif_release(netif);
         return res;
     }
 #else  /* CONFIG_GNRC_IPV6_NIB_ARSM */