Silence GCC warnings in dhcp.c

The first was a potential out of range index read in dhcp_handle_ack().
The (n < DNS_MAX_SERVERS) check should occur first.  There is also a
documented lwIP bug for this issue here:
    http://savannah.nongnu.org/bugs/?36170

In dhcp_bind() there is no need to perform the NULL check in
ip_addr_isany() for &gw_addr.  Just check (gw_addr.addr == IPADDR_ANY)
instead.

I refactored the chaddr[] copy in dhcp_create_msg() to first copy all
of the valid bytes in hwaddr and then pad the rest of the bytes with 0.
Before it used to check on every destination byte if it should copy or
pad.  GCC originally complained about an index out of range read from
the hwaddr[] array even though it was protected by a conditional
operator.  The refactor makes the intent a bit clearer and saves the
extra comparison per loop iteration.  It also stops GCC from
complaining :)

Author: adamgreen

libraries/net/lwip/lwip/core/dhcp.c

@@ -564,7 +564,7 @@ dhcp_handle_ack(struct netif *netif)
 #if LWIP_DNS
   /* DNS servers */
   n = 0;
-  while(dhcp_option_given(dhcp, DHCP_OPTION_IDX_DNS_SERVER + n) && (n < DNS_MAX_SERVERS)) {
+  while((n < DNS_MAX_SERVERS) && dhcp_option_given(dhcp, DHCP_OPTION_IDX_DNS_SERVER + n)) {
     ip_addr_t dns_addr;
     ip4_addr_set_u32(&dns_addr, htonl(dhcp_get_option_value(dhcp, DHCP_OPTION_IDX_DNS_SERVER + n)));
     dns_setserver(n, &dns_addr);
@@ -975,7 +975,7 @@ dhcp_bind(struct netif *netif)
 
   ip_addr_copy(gw_addr, dhcp->offered_gw_addr);
   /* gateway address not given? */
-  if (ip_addr_isany(&gw_addr)) {
+  if (gw_addr.addr == IPADDR_ANY) {
     /* copy network address */
     ip_addr_get_network(&gw_addr, &dhcp->offered_ip_addr, &sn_mask);
     /* use first host address on network as gateway */
@@ -1678,9 +1678,13 @@ dhcp_create_msg(struct netif *netif, struct dhcp *dhcp, u8_t message_type)
   ip_addr_set_zero(&dhcp->msg_out->yiaddr);
   ip_addr_set_zero(&dhcp->msg_out->siaddr);
   ip_addr_set_zero(&dhcp->msg_out->giaddr);
-  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*/;
+  for (i = 0; i < netif->hwaddr_len; i++) {
+    /* copy netif hardware address */
+    dhcp->msg_out->chaddr[i] = netif->hwaddr[i];
+  }
+  for ( ; i < DHCP_CHADDR_LEN; i++) {
+    /* ... pad rest with zeroes */
+    dhcp->msg_out->chaddr[i] = 0;
   }
   for (i = 0; i < DHCP_SNAME_LEN; i++) {
     dhcp->msg_out->sname[i] = 0;