stubdom: fix lwip compile

stubdom: fix lwip compile

stubdom/lwip-x86_64/src/core/dhcp.c: In function 'dhcp_create_request':
stubdom/lwip-x86_64/src/core/dhcp.c:1359:71: error: array subscript is above array bounds [-Werror=array-bounds]
     dhcp->msg_out->chaddr[i] = (i < netif->hwaddr_len) ? netif->hwaddr[i] : 0/* pad byte*/;

gcc can not know if hwaddr_len exceeds the hwaddr array size,
so force an upper limit to assist gcc.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>