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

Fixes #1104

flutter/sherpa_onnx/lib/src/utils.dart

@@ -0,0 +1,25 @@
+// 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);
+}