Pull request: fix zero-length ip addresses list bug

Merge in DNS/adguard-home from 2304-fix-panic to master

Updates #2304.

Squashed commit of the following:

commit bd7742eb144b46e16c751f98f6a4a6f15fbfa60e
Merge: 26313926e 7d1d87d6e
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Wed Dec 2 14:29:37 2020 +0300

    Merge branch 'master' into 2304-fix-panic

commit 26313926e827d1f5ceb4eec744b814ce7c32663d
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Wed Dec 2 14:09:16 2020 +0300

    all: add gitignore rule

commit 5a8521bd9b4014972107e8de352e20144f8187fb
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Wed Dec 2 14:03:26 2020 +0300

    dhcpd: fix zero-length ip addresses list bug

.gitignore

@@ -12,6 +12,7 @@
 /querylog.json
 /querylog.json.1
 coverage.txt
+leases.db
 
 # Test output
 dnsfilter/tests/top-1m.csv

internal/dhcpd/v4.go

@@ -581,6 +581,11 @@ func (s *v4Server) Start() error {
 		return fmt.Errorf("dhcpv4: interface %s: %w", ifaceName, err)
 	}
 
+	if len(dnsIPAddrs) == 0 {
+		// No available IP addresses which may appear later.
+		return nil
+	}
+
 	s.conf.dnsIPAddrs = dnsIPAddrs
 
 	laddr := &net.UDPAddr{

internal/dhcpd/v6.go

@@ -579,6 +579,11 @@ func (s *v6Server) Start() error {
 		return fmt.Errorf("dhcpv6: interface %s: %w", ifaceName, err)
 	}
 
+	if len(dnsIPAddrs) == 0 {
+		// No available IP addresses which may appear later.
+		return nil
+	}
+
 	s.conf.dnsIPAddrs = dnsIPAddrs
 
 	err = s.initRA(iface)