ext/snmp: add test for GH-21341 suffix-as-keys snprintf loop

Signed-off-by: Thomas Vincent <thomasvincent@gmail.com>

Author: somethingwithproof

ext/snmp/tests/gh21341.phpt

@@ -0,0 +1,37 @@
+--TEST--
+GH-21341: suffix-as-keys walk uses offset-tracked snprintf (no strcat overflow)
+--EXTENSIONS--
+snmp
+--SKIPIF--
+<?php
+require_once(__DIR__.'/skipif.inc');
+if (getenv('SKIP_ASAN')) die('skip Timeouts under ASAN');
+?>
+--FILE--
+<?php
+require_once(__DIR__.'/snmp_include.inc');
+
+/* Exercise the suffix-as-keys code path (SNMP_USE_SUFFIX_AS_KEYS |
+ * SNMP_CMD_WALK).  The walk OID is chosen so the returned subidentifiers
+ * form a multi-component suffix, driving multiple iterations of the
+ * snprintf loop that GH-21341 replaced the strcat loop with. */
+$session = new SNMP(SNMP::VERSION_2c, $hostname, $community, $timeout, $retries);
+$result = $session->walk('.1.3.6.1.2.1.1', /* suffix_as_keys */ true);
+
+var_dump(is_array($result));
+var_dump(count($result) > 0);
+
+/* Every key must be a bare dotted-numeric suffix, not a full OID.
+ * Full OIDs start with '.'; suffixes do not. */
+$bad = array_filter(array_keys($result), static function (string $k): bool {
+    return $k[0] === '.' || !preg_match('/^\d[\d.]*$/', $k);
+});
+var_dump(count($bad) === 0);
+
+var_dump($session->close());
+?>
+--EXPECT--
+bool(true)
+bool(true)
+bool(true)
+bool(true)