- 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.

home/dns.go

@@ -62,10 +62,12 @@ func initDNSServer() error {
 	Context.dnsFilter = dnsfilter.New(&filterConf, nil)
 
 	p := dnsforward.DNSCreateParams{
-		DNSFilter:  Context.dnsFilter,
-		Stats:      Context.stats,
-		QueryLog:   Context.queryLog,
-		DHCPServer: Context.dhcpServer,
+		DNSFilter: Context.dnsFilter,
+		Stats:     Context.stats,
+		QueryLog:  Context.queryLog,
+	}
+	if Context.dhcpServer != nil {
+		p.DHCPServer = Context.dhcpServer
 	}
 	Context.dnsServer = dnsforward.NewServer(p)
 	dnsConfig := generateServerConfig()