Fix free of uninitialized memory in attributes

Fixes OSS-Fuzz #23140.

Author: nikic

Zend/tests/attributes/018_fatal_error_in_argument.phpt

@@ -0,0 +1,11 @@
+--TEST--
+Don't free uninitialized memory if a fatal error occurs in an attribute argument
+--FILE--
+<?php
+
+<<Attr(a->b::c)>>
+function test() {}
+
+?>
+--EXPECTF--
+Fatal error: Dynamic class names are not allowed in compile-time class constant references in %s on line %d

Zend/zend_attributes.c

@@ -113,6 +113,11 @@ ZEND_API zend_attribute *zend_add_attribute(HashTable **attributes, zend_bool pe
 	attr->offset = offset;
 	attr->argc = argc;
 
+	/* Initialize arguments to avoid partial initialization in case of fatal errors. */
+	for (uint32_t i = 0; i < argc; i++) {
+		ZVAL_UNDEF(&attr->argv[i]);
+	}
+
 	zend_hash_next_index_insert_ptr(*attributes, attr);
 
 	return attr;