Fix symtable cache being used while cleaning symtable

We need to first clean the symtable and then check whether a cache
slot is available for it. Otherwise, it may happen that a destructor
runs while cleaning the table and uses up all the remaining slots
in the cache.

This is particularly insidious because once we overflow the cache,
the first pointer we modify is symtable_cache_ptr, making it hard
to understand what happened after the fact.

Fixes oss-fuzz #30815.

Author: nikic

Zend/tests/symtable_cache_recursive_dtor.phpt

@@ -0,0 +1,19 @@
+--TEST--
+Symtable cache slots may be acquired while cleaning symtable
+--FILE--
+<?php
+class A {
+    // Must be larger than the symtable cache.
+    static $max = 40;
+    function __destruct() {
+        if (self::$max-- < 0) return;
+        $x = 'y';
+        $$x = new a;
+    }
+}
+new A;
+
+?>
+===DONE===
+--EXPECT--
+===DONE===

Zend/zend_execute.c

@@ -3437,12 +3437,13 @@ ZEND_API void execute_internal(zend_execute_data *execute_data, zval *return_val
 
 ZEND_API void zend_clean_and_cache_symbol_table(zend_array *symbol_table) /* {{{ */
 {
+	/* Clean before putting into the cache, since clean could call dtors,
+	 * which could use the cached hash. Also do this before the check for
+	 * available cache slots, as those may be used by a dtor as well. */
+	zend_symtable_clean(symbol_table);
 	if (EG(symtable_cache_ptr) >= EG(symtable_cache_limit)) {
 		zend_array_destroy(symbol_table);
 	} else {
-		/* clean before putting into the cache, since clean
-		   could call dtors, which could use cached hash */
-		zend_symtable_clean(symbol_table);
 		*(EG(symtable_cache_ptr)++) = symbol_table;
 	}
 }