cherry-pick: 3987 Fix nil pointer dereference

Merge in DNS/adguard-home from 3987-fix-nil-deref to master

Updates #3987.
Updates #2846.

Squashed commit of the following:

commit d653e09ce88a8b10b2a17fea1563c419895c714c
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Dec 23 20:08:51 2021 +0300

    all: log changes

commit c47a4eeacf76fa7df2d01af166dee9d52528ac58
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Dec 23 19:22:39 2021 +0300

    aghnet: fix windows tests

commit 9c91f14ccfe967ada3c00ddb86d673238e52c12d
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Dec 23 19:09:49 2021 +0300

    aghnet: imp code readability, docs

commit d3df15d1892e4ebfe7f8ea7144e39a0c712fce52
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Dec 23 18:47:28 2021 +0300

    aghnet: fix nil pointer dereference

internal/aghnet/net_windows.go

@@ -25,6 +25,7 @@ func ifaceSetStaticIP(string) (err error) {
 	return aghos.Unsupported("setting static ip")
 }
 
+// closePortChecker closes c.  c must be non-nil.
 func closePortChecker(c io.Closer) (err error) {
 	if err = c.Close(); err != nil {
 		return err