[dhcp] Do not transition to DHCPREQUEST without a valid DHCPOFFER

A missing test for dhcp->dhcpoffer in dhcp_timer_expired() was causing
the client to transition to DHCPREQUEST after timing out on waiting
for ProxyDHCP even if no DHCPOFFERs had been received.

src/net/udp/dhcp.c

 	}
 
 	/* Give up waiting for ProxyDHCP before we reach the failure point */
-	if ( elapsed > PROXYDHCP_WAIT_TIME ) {
+	if ( dhcp->dhcpoffer && ( elapsed > PROXYDHCP_WAIT_TIME ) ) {
 		if ( dhcp->state == DHCP_STATE_DISCOVER ) {
 			dhcp_set_state ( dhcp, DHCP_STATE_REQUEST );
 			return;