Fix use-after-free serializing an array grown by an element's hook

The IS_ARRAY case of php_var_serialize_intern() walked the array's
HashTable without holding a reference across
php_var_serialize_nested_data(), which recurses into user hooks
(__serialize, __sleep, Serializable::serialize). A hook that grows the
same array through a by-reference alias reallocs the backing store
mid-walk, so the iterator reads freed memory. Hold a ref across the walk,
as the object path and var_dump/var_export already do, so the append
separates a copy instead of reallocating in place.

Author: iliaal

ext/standard/tests/serialize/serialize_array_ref_reentrancy.phpt

@@ -0,0 +1,21 @@
+--TEST--
+serialize(): a by-reference __serialize() that grows the array being walked must not free it
+--FILE--
+<?php
+class G {
+    public $ref;
+    public function __serialize(): array {
+        for ($i = 0; $i < 128; $i++) {
+            $this->ref[] = 'x' . $i;
+        }
+        return ['d' => 1];
+    }
+}
+$g = new G();
+$inner = [$g, 'tail'];
+$g->ref = &$inner;
+$top = [&$inner];
+var_dump(serialize($top));
+?>
+--EXPECT--
+string(59) "a:1:{i:0;a:2:{i:0;O:1:"G":1:{s:1:"d";i:1;}i:1;s:4:"tail";}}"

ext/standard/var.c

@@ -1301,13 +1301,21 @@ static void php_var_serialize_intern(smart_str *buf, zval *struc, php_serialize_
 				zend_release_properties(myht);
 				return;
 			}
-		case IS_ARRAY:
+		case IS_ARRAY: {
 			smart_str_appendl(buf, "a:", 2);
 			myht = Z_ARRVAL_P(struc);
+			bool rcn = !is_root && (in_rcn_array || GC_REFCOUNT(myht) > 1);
+			if (!(GC_FLAGS(myht) & GC_IMMUTABLE)) {
+				GC_ADDREF(myht);
+			}
 			php_var_serialize_nested_data(
 				buf, struc, myht, zend_array_count(myht), /* incomplete_class */ 0, var_hash,
-					!is_root && (in_rcn_array || GC_REFCOUNT(myht) > 1));
+					rcn);
+			if (!(GC_FLAGS(myht) & GC_IMMUTABLE)) {
+				GC_DTOR_NO_REF(myht);
+			}
 			return;
+		}
 		case IS_REFERENCE:
 			struc = Z_REFVAL_P(struc);
 			goto again;