resolve: return to UDP if removing lame nameserver

lib/resolve.c

@@ -111,8 +111,9 @@ static int iterate(struct knot_requestor *requestor, struct kr_layer_param *para
 			cur->flags |= QUERY_TCP;
 			return iterate(requestor, param);
 		}
-		/* Resolution failed, invalidate current NS. */
+		/* Resolution failed, invalidate current NS and reset to UDP. */
 		ret = invalidate_ns(rplan, cur);
+		cur->flags &= ~QUERY_TCP;
 	}
 
 	return ret;

tests/testdata/iter_minim_ns.rpl

@@ -32,12 +32,21 @@ REPLY QR NOERROR
 SECTION QUESTION
 com. IN NS
 SECTION AUTHORITY
+com.	IN NS	lame-addr.gtld-servers.net.
 com.	IN NS	a.gtld-servers.net.
 SECTION ADDITIONAL
+lame-addr.gtld-servers.net.	IN 	A	1.1.1.1
+a.gtld-servers.net.	IN 	A	1.1.1.1
 a.gtld-servers.net.	IN 	A	192.5.6.30
 ENTRY_END
 RANGE_END
 
+; lame.gtld-servers.net.
+RANGE_BEGIN 0 100
+	ADDRESS 1.1.1.1
+
+EANGE_END
+
 ; a.gtld-servers.net.
 RANGE_BEGIN 0 100
 	ADDRESS 192.5.6.30