Handle invalid utf8 sequence from Whisper for Dart API. (#1106)

Fixes #1104

+## 1.10.14 (to-be-released)
+
+* Fix invalid utf8 sequence from Whisper for Dart API.
+
 ## 1.10.13
 
 * Update onnxruntime from 1.17.1 to 1.18.0

@@ -7,6 +7,7 @@ import 'package:ffi/ffi.dart';
 import './feature_config.dart';
 import './offline_stream.dart';
 import './sherpa_onnx_bindings.dart';
+import './utils.dart';
 
 class OfflineTransducerModelConfig {
   const OfflineTransducerModelConfig({
@@ -287,7 +288,7 @@ class OfflineRecognizer {
       return OfflineRecognizerResult(text: '', tokens: [], timestamps: []);
     }
 
-    final parsedJson = jsonDecode(json.toDartString());
+    final parsedJson = jsonDecode(toDartString(json));
 
     SherpaOnnxBindings.destroyOfflineStreamResultJson?.call(json);
 

@@ -7,6 +7,7 @@ import 'package:ffi/ffi.dart';
 import './feature_config.dart';
 import './online_stream.dart';
 import './sherpa_onnx_bindings.dart';
+import './utils.dart';
 
 class OnlineTransducerModelConfig {
   const OnlineTransducerModelConfig({
@@ -268,7 +269,7 @@ class OnlineRecognizer {
       return OnlineRecognizerResult(text: '', tokens: [], timestamps: []);
     }
 
-    final parsedJson = jsonDecode(json.toDartString());
+    final parsedJson = jsonDecode(toDartString(json));
 
     SherpaOnnxBindings.destroyOnlineStreamResultJson?.call(json);
 

+// Copyright (c)  2024  Xiaomi Corporation
+import 'dart:convert';
+import 'dart:ffi';
+import 'dart:typed_data';
+
+import 'package:ffi/ffi.dart';
+
+int _strLen(Pointer<Uint8> codeUnits) {
+  // this function is copied from
+  // https://github.com/dart-archive/ffi/blob/main/lib/src/utf8.dart#L52
+  var length = 0;
+  while (codeUnits[length] != 0) {
+    length++;
+  }
+  return length;
+}
+
+// This function is modified from
+// https://github.com/dart-archive/ffi/blob/main/lib/src/utf8.dart#L41
+// It ignores invalid utf8 sequence
+String toDartString(Pointer<Utf8> s) {
+  final codeUnits = s.cast<Uint8>();
+  final length = _strLen(codeUnits);
+  return utf8.decode(codeUnits.asTypedList(length), allowMalformed: true);
+}