[dhcp] Do not skip ProxyDHCPREQUEST if next-server is empty

We attempt to mimic the behaviour of Intel's PXE ROM by skipping the
separate ProxyDHCPREQUEST if the ProxyDHCPOFFER already contains a
boot filename or a PXE boot menu.

Experimentation reveals that Intel's PXE ROM will also check for a
non-empty next-server address alongside the boot filename.  Update our
test to match this behaviour.

Reported-by: Wissam Shoukair <wissams@mellanox.com>
Tested-by: Wissam Shoukair <wissams@mellanox.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/udp/dhcp.c

  */
 static int dhcp_has_pxeopts ( struct dhcp_packet *dhcppkt ) {
 
-	/* Check for a boot filename */
-	if ( dhcppkt_fetch ( dhcppkt, DHCP_BOOTFILE_NAME, NULL, 0 ) > 0 )
+	/* Check for a next-server and boot filename */
+	if ( dhcppkt->dhcphdr->siaddr.s_addr &&
+	     ( dhcppkt_fetch ( dhcppkt, DHCP_BOOTFILE_NAME, NULL, 0 ) > 0 ) )
 		return 1;
 
 	/* Check for a PXE boot menu */