sys/net/dhcpv6: consider option 0 an end marker

The REPLY of a TP-Link router (WR400 v4.20) ends with a 0 option
that has a bogus length.
When subtracting the length from the remaining length, we get an
underflow (remaining `len` is 3, option len is 4).

To get a working DHCP response, consider 0 (Reserved) an end marker.

sys/net/application_layer/dhcpv6/client.c

@@ -1016,6 +1016,10 @@ static bool _parse_reply(uint8_t *rep, size_t len, uint8_t request_type)
             default:
                 break;
         }
+        /* 0 option is used as an end marker, len can include bogus bytes */
+        if (!byteorder_ntohs(opt->type)) {
+            break;
+        }
     }
     if ((cid == NULL) || (sid == NULL)) {
         DEBUG("DHCPv6 client: ADVERTISE does not contain either server ID "