Fix use of magic const within const expr cast (GH-18338)

Fixes OSS-Fuzz #410939023

Author: iluuu1994

Zend/tests/oss_fuzz_410939023.phpt

@@ -0,0 +1,11 @@
+--TEST--
+OSS-Fuzz #410939023: Use of magic const within const expr cast
+--FILE--
+<?php
+
+const C = (string)__METHOD__;
+var_dump(C);
+
+?>
+--EXPECT--
+string(0) ""

Zend/zend_compile.c

@@ -12127,6 +12127,9 @@ static void zend_eval_const_expr(zend_ast **ast_ptr) /* {{{ */
 			zend_eval_const_expr(&ast->child[0]);
 			zend_eval_const_expr(&ast->child[1]);
 			return;
+		case ZEND_AST_CAST:
+			zend_eval_const_expr(&ast->child[0]);
+			return;
 		default:
 			return;
 	}