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>

stubdom/Makefile

@@ -145,6 +145,7 @@ lwip-$(XEN_TARGET_ARCH): lwip-$(LWIP_VERSION).tar.gz
 	tar xzf $<
 	mv lwip $@
 	patch -d $@ -p0 < lwip.patch-cvs
+	patch -d $@ -p0 < lwip.dhcp_create_request-hwaddr_len.patch
 	touch $@
 
 #############

stubdom/lwip.dhcp_create_request-hwaddr_len.patch

@@ -0,0 +1,26 @@
+---
+ lwip-x86_64/src/core/dhcp.c |    4 +++-
+ 1 file changed, 3 insertions(+), 1 deletion(-)
+
+Index: src/core/dhcp.c
+===================================================================
+--- src/core/dhcp.c
++++ src/core/dhcp.c
+@@ -1322,6 +1322,8 @@ dhcp_create_request(struct netif *netif)
+ {
+   struct dhcp *dhcp;
+   u16_t i;
++  /* gcc can not know if hwaddr_len exceeds the hwaddr array size */
++  u8_t hwaddr_len = netif->hwaddr_len > NETIF_MAX_HWADDR_LEN ? NETIF_MAX_HWADDR_LEN : netif->hwaddr_len;
+   LWIP_ERROR("dhcp_create_request: netif != NULL", (netif != NULL), return ERR_ARG;);
+   dhcp = netif->dhcp;
+   LWIP_ERROR("dhcp_create_request: dhcp != NULL", (dhcp != NULL), return ERR_VAL;);
+@@ -1356,7 +1358,7 @@ dhcp_create_request(struct netif *netif)
+   dhcp->msg_out->giaddr.addr = 0;
+   for (i = 0; i < DHCP_CHADDR_LEN; i++) {
+     /* copy netif hardware address, pad with zeroes */
+-    dhcp->msg_out->chaddr[i] = (i < netif->hwaddr_len) ? netif->hwaddr[i] : 0/* pad byte*/;
++    dhcp->msg_out->chaddr[i] = (i < hwaddr_len) ? netif->hwaddr[i] : 0/* pad byte*/;
+   }
+   for (i = 0; i < DHCP_SNAME_LEN; i++) {
+     dhcp->msg_out->sname[i] = 0;