fix: fix iter unicode string (#1802)

Signed-off-by: he1pa <[email protected]>

kclvm/evaluator/src/snapshots/kclvm_evaluator__tests__list_comp1.snap

@@ -0,0 +1,7 @@
+---
+source: evaluator/src/tests.rs
+expression: "format! (\"{}\", evaluator.run().unwrap().1)"
+---
+a:
+- 你
+- 好

kclvm/evaluator/src/tests.rs

@@ -478,6 +478,10 @@ data2 = Data {
 version = "v0.1.0"
 "#}
 
+evaluator_snapshot! {list_comp1, r#"
+a = [ x for x in "你好"]
+"#}
+
 #[test]
 fn test_if_stmt_setters() {
     let p = load_packages(&LoadPackageOptions {

kclvm/runtime/src/value/iter.rs

@@ -113,6 +113,9 @@ impl ValueIterator {
         }
         match *host.rc.borrow() {
             Value::str_value(ref s) => {
+                if self.pos >= s.chars().count() as i32 {
+                    return None;
+                }
                 let ch = s.chars().nth(self.pos as usize).unwrap();
                 self.cur_key = ValueRef::int(self.pos as i64);
                 self.cur_val = ValueRef::str(&ch.to_string());