- windows: install: fix crash due to empty DHCP server pointer

#2036

Squashed commit of the following:

commit 5c70d28b241d1a98e034f6798dade073b1b42511
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Mon Aug 24 18:42:21 2020 +0300

    - windows: install: fix crash due to empty DHCP server pointer

    We should not assign a Go interface value, otherwise `==nil` check doesn't work.

dnsforward/dnsforward.go

@@ -82,9 +82,9 @@ func NewServer(p DNSCreateParams) *Server {
 	s.dnsFilter = p.DNSFilter
 	s.stats = p.Stats
 	s.queryLog = p.QueryLog
-	s.dhcpServer = p.DHCPServer
 
-	if s.dhcpServer != nil {
+	if p.DHCPServer != nil {
+		s.dhcpServer = p.DHCPServer
 		s.dhcpServer.SetOnLeaseChanged(s.onDHCPLeaseChanged)
 		s.onDHCPLeaseChanged(dhcpd.LeaseChangedAdded)
 	}