gnrc_ipv6: remove superfluous check for if_entry

sys/net/gnrc/network_layer/ipv6/gnrc_ipv6.c

@@ -341,7 +341,7 @@ static void _send_to_iface(kernel_pid_t iface, gnrc_pktsnip_t *pkt)
         return;
     }
 #ifdef MODULE_GNRC_SIXLOWPAN
-    if ((if_entry != NULL) && (if_entry->flags & GNRC_IPV6_NETIF_FLAGS_SIXLOWPAN)) {
+    if (if_entry->flags & GNRC_IPV6_NETIF_FLAGS_SIXLOWPAN) {
         DEBUG("ipv6: send to 6LoWPAN instead\n");
         if (!gnrc_netapi_dispatch_send(GNRC_NETTYPE_SIXLOWPAN, GNRC_NETREG_DEMUX_CTX_ALL, pkt)) {
             DEBUG("ipv6: no 6LoWPAN thread found");