gnrc_ipv6_nib: release packet when NC entry can't be added

When the neighbor cache is full, the packet trying to resolve a
neighbor is not released.

This change fixes that.

sys/net/gnrc/network_layer/ipv6/nib/nib.c

@@ -1130,6 +1130,7 @@ static bool _resolve_addr(const ipv6_addr_t *dst, gnrc_netif_t *netif,
             entry = _nib_nc_add(dst, (netif != NULL) ? netif->pid : 0,
                                 GNRC_IPV6_NIB_NC_INFO_NUD_STATE_INCOMPLETE);
             if (entry == NULL) {
+                gnrc_pktbuf_release(pkt);
                 return false;
             }
 #if GNRC_IPV6_NIB_CONF_ROUTER