[dhcp] Ignore ProxyDHCPACKs without PXE options

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

src/net/udp/dhcp.c

 		DBGC ( dhcp, " (%s/", inet_ntoa ( server_id ) );
 		DBGC ( dhcp, "%s)", inet_ntoa ( pseudo_id ) );
 	}
+	if ( dhcp_has_pxeopts ( dhcppkt ) )
+		DBGC ( dhcp, " pxe" );
 	DBGC ( dhcp, "\n" );
 
 	/* Filter out unacceptable responses */
 		return;
 	if ( ( pseudo_id.s_addr != dhcp->proxy_server.s_addr ) )
 		return;
+	if ( ! dhcp_has_pxeopts ( dhcppkt ) )
+		return;
 
 	/* Register settings */
 	if ( ( rc = register_settings ( settings, NULL,