Pull request: dnsforward: fix infinite loop

Merge in DNS/adguard-home from 2619-infinite-loop to master

Updates #2619.

Squashed commit of the following:

commit 425fde3f476349571036b9b95c9d0a33703270ee
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Feb 2 13:08:52 2021 +0300

    dnsforward: fix infinite loop

internal/dnsforward/ipset_linux.go

@@ -242,6 +242,9 @@ func (c *ipsetCtx) lookupHost(host string) (sets []ipsetProps) {
 		}
 
 		i = strings.Index(host, ".")
+		if i == -1 {
+			break
+		}
 	}
 
 	// Check the root catch-all one.