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

CHANGELOG.md

@@ -10,9 +10,38 @@ and this project adheres to
 ## [Unreleased]
 
 <!--
-## [v0.107.1] - 2022-01-25 (APPROX.)
+## [v0.108.0] - 2021-06-01 (APPROX.)
 -->
 
+### Added
+
+- `windows/arm64` support ([#3057]).
+
+### Deprecated
+
+- Go 1.17 support.  v0.109.0 will require at least Go 1.18 to build.
+
+### Removed
+
+- Go 1.16 support.
+
+[#3057]: https://github.com/AdguardTeam/AdGuardHome/issues/3057
+
+
+
+<!--
+## [v0.107.1] - 2021-12-29 (APPROX.)
+-->
+## v0.107.1 - UNRELEASED
+
+### Fixed
+
+- Panic on port availability check during installation ([#3987]).
+
+[#3987]: https://github.com/AdguardTeam/AdGuardHome/issues/3987
+
+
+
 ## [v0.107.0] - 2021-12-21
 
 ### Added

internal/aghnet/net.go

@@ -187,7 +187,8 @@ func GetSubnet(ifaceName string) *net.IPNet {
 	return nil
 }
 
-// CheckPort checks if the port is available for binding.
+// CheckPort checks if the port is available for binding.  network is expected
+// to be one of "udp" and "tcp".
 func CheckPort(network string, ip net.IP, port int) (err error) {
 	var c io.Closer
 	addr := netutil.IPPort{IP: ip, Port: port}.String()
@@ -200,7 +201,11 @@ func CheckPort(network string, ip net.IP, port int) (err error) {
 		return nil
 	}
 
-	return errors.WithDeferred(err, closePortChecker(c))
+	if err != nil {
+		return err
+	}
+
+	return closePortChecker(c)
 }
 
 // IsAddrInUse checks if err is about unsuccessful address binding.

internal/aghnet/net_test.go

@@ -5,6 +5,8 @@ import (
 	"testing"
 
 	"github.com/AdguardTeam/AdGuardHome/internal/aghtest"
+	"github.com/AdguardTeam/golibs/netutil"
+	"github.com/AdguardTeam/golibs/testutil"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -69,3 +71,20 @@ func TestBroadcastFromIPNet(t *testing.T) {
 		})
 	}
 }
+
+func TestCheckPort(t *testing.T) {
+	l, err := net.Listen("tcp", "127.0.0.1:")
+	require.NoError(t, err)
+	testutil.CleanupAndRequireSuccess(t, l.Close)
+
+	ipp := netutil.IPPortFromAddr(l.Addr())
+	require.NotNil(t, ipp)
+	require.NotNil(t, ipp.IP)
+	require.NotZero(t, ipp.Port)
+
+	err = CheckPort("tcp", ipp.IP, ipp.Port)
+	target := &net.OpError{}
+	require.ErrorAs(t, err, &target)
+
+	assert.Equal(t, "listen", target.Op)
+}

internal/aghnet/net_unix.go

@@ -10,6 +10,7 @@ import (
 	"github.com/AdguardTeam/golibs/errors"
 )
 
+// closePortChecker closes c.  c must be non-nil.
 func closePortChecker(c io.Closer) (err error) {
 	return c.Close()
 }

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