Fix segfault with __COMPILER_HALT_OFFSET__ and trailing {}

Fixes OSS-Fuzz #17895.

Author: nikic

Zend/tests/halt_compiler5.phpt

@@ -0,0 +1,8 @@
+--TEST--
+Using __COMPILER_HALF_OFFSET__ with trailing {} (OSS-Fuzz #17895)
+--FILE--
+<?php
+__COMPILER_HALT_OFFSET__;
+{}
+--EXPECTF--
+Warning: Use of undefined constant __COMPILER_HALT_OFFSET__ - assumed '__COMPILER_HALT_OFFSET__' (this will throw an Error in a future version of PHP) in %s on line %d

Zend/zend_compile.c

@@ -7676,6 +7676,9 @@ void zend_compile_const(znode *result, zend_ast *ast) /* {{{ */
 
 		while (last && last->kind == ZEND_AST_STMT_LIST) {
 			zend_ast_list *list = zend_ast_get_list(last);
+			if (list->children == 0) {
+				break;
+			}
 			last = list->child[list->children-1];
 		}
 		if (last && last->kind == ZEND_AST_HALT_COMPILER) {