Fix crash on (unset) cast in constant property initializer

Author: arshidkv12

Zend/tests/ast/gh21072.phpt

@@ -0,0 +1,15 @@
+--TEST--
+(unset) cast must not be allowed in constant property initializer
+--FILE--
+<?php
+try {
+    class C {
+        public $p = (unset) C::class;
+    }
+    new C;
+} catch (Error $e) {
+    echo $e->getMessage();
+}
+?>
+--EXPECT--
+Invalid cast in constant expression

Zend/zend_ast.c

@@ -814,6 +814,11 @@ static zend_result ZEND_FASTCALL zend_ast_evaluate_inner(
 					case IS_OBJECT:
 						zend_cast_zval_to_object(result, &op1, IS_VAR);
 						break;
+					case IS_NULL:
+						zend_throw_error(NULL,
+							"Invalid cast in constant expression");
+						ret = FAILURE;
+						break;
 					EMPTY_SWITCH_DEFAULT_CASE();
 				}
 				zval_ptr_dtor_nogc(&op1);