fix negative frankenphp_ready_workers metrics (#1491)

testdata/worker-restart.php

@@ -0,0 +1,13 @@
+<?php
+
+$fn = static function () {
+    echo sprintf("Counter (%s)", $_GET['i'] ?? 'i not set');
+};
+
+$loopMax = $_SERVER['EVERY'] ?? 10;
+$loops = 0;
+do {
+    $ret = \frankenphp_handle_request($fn);
+} while ($ret && (-1 === $loopMax || ++$loops < $loopMax));
+
+exit(0);