Fix crash on (unset) cast in constant property initializer

Zend/tests/ast/gh21072.phpt

@@ -0,0 +1,15 @@
+--TEST--
+(unset) cast must not be allowed in constant expressions
+--FILE--
+<?php
+try {
+    class C {
+        public $p = (unset) C::class;
+    }
+    new C;
+} catch (Error $e) {
+    echo $e->getMessage();
+}
+?>
+--EXPECTF--
+Fatal error: The (unset) cast is no longer supported in %s on line %d

Zend/zend_compile.c

@@ -12565,6 +12565,9 @@ static void zend_eval_const_expr(zend_ast **ast_ptr) /* {{{ */
 			return;
 		case ZEND_AST_CAST:
 			zend_eval_const_expr(&ast->child[0]);
+			if (ast->attr == IS_NULL) {
+				zend_error_noreturn(E_COMPILE_ERROR, "The (unset) cast is no longer supported");
+			}
 			if (ast->child[0]->kind == ZEND_AST_ZVAL
 			 && zend_try_ct_eval_cast(&result, ast->attr, zend_ast_get_zval(ast->child[0]))) {
 				break;