dig: always set the default port when doing a UDP query

This commit ensures that the port is set before attempting a UDP
query. Before that a situation could appear when previous query have
completed over a different transport (that uses a dedicated port) and
then a UDP query will be attempted over the port of the previous
transport.

(cherry picked from commit e390ed4421)

bin/dig/dighost.c

@@ -3308,6 +3308,10 @@ start_udp(dig_query_t *query) {
 		return;
 	}
 
+	if (!port_set) {
+		port = 53;
+	}
+
 	result = get_address(query->servname, port, &query->sockaddr);
 	if (result != ISC_R_SUCCESS) {
 		/* This servname doesn't have an address. */