Re-add the comment to streamdns_readmore()

Put the comment back, so it's more obvious that we are only restarting
timer when there's a last handle attached to the socket; there has to be
always at least one.

lib/isc/netmgr/streamdns.c

@@ -103,8 +103,10 @@ static void
 streamdns_readmore(isc_nmsocket_t *sock, isc_nmhandle_t *transphandle) {
 	streamdns_resumeread(sock, transphandle);
 
+	/* Restart the timer only if there's a last single active handle */
 	isc_nmhandle_t *handle = ISC_LIST_HEAD(sock->active_handles);
-	if (handle != NULL && ISC_LIST_NEXT(handle, active_link) == NULL) {
+	INSIST(handle != NULL);
+	if (ISC_LIST_NEXT(handle, active_link) == NULL) {
 		isc__nmsocket_timer_start(sock);
 	}
 }