Pull request:* all: fix all staticcheck SA warnings

Merge in DNS/adguard-home from 2238-fix-static-analisys-warnings to master

Squashed commit of the following:

commit 721ca6fa1cbfdfe9d414e6ed52fec4a64653fb52
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Oct 30 15:48:10 2020 +0300

    * all: fix all staticcheck SA warnings

    Closes #2238.

internal/dhcpd/dhcp_http.go

@@ -175,7 +175,7 @@ func (s *Server) handleDHCPSetConfig(w http.ResponseWriter, r *http.Request) {
 		v6conf.InterfaceName = newconfig.InterfaceName
 		v6conf.notify = s.onNotify
 		s6, err = v6Create(v6conf)
-		if s6 == nil {
+		if err != nil {
 			httpError(r, w, http.StatusBadRequest, "Invalid DHCPv6 configuration: %s", err)
 			return
 		}