Fix +1 problem in LocalBundleReader (#10656)

* Fix +1 problem in LocalBundleReader

* Change files

change/react-native-windows-d5e1c0c0-cb20-47fd-947a-eb449f7e9677.json

@@ -0,0 +1,7 @@
+{
+  "type": "none",
+  "comment": "Fix +1 problem in LocalBundleReader",
+  "packageName": "react-native-windows",
+  "email": "aeulitz@microsoft.com",
+  "dependentChangeType": "none"
+}

vnext/Microsoft.ReactNative/Utils/LocalBundleReader.cpp

@@ -76,14 +76,15 @@ std::future<std::string> LocalBundleReader::LoadBundleAsync(const std::string &b
   auto fileBuffer{co_await winrt::Windows::Storage::FileIO::ReadBufferAsync(file)};
   auto dataReader{winrt::Windows::Storage::Streams::DataReader::FromBuffer(fileBuffer)};
 
-  std::string script(fileBuffer.Length() + 1, '\0');
+  // No need to use length + 1, STL guarantees that string storage is null-terminated.
+  std::string script(fileBuffer.Length(), '\0');
 
   // Construct the array_view to slice into the first fileBuffer.Length bytes.
   // DataReader.ReadBytes will read as many bytes as are present in the
   // array_view. The backing string has fileBuffer.Length() + 1 bytes, without
   // an explicit end it will read 1 byte to many and throw.
   dataReader.ReadBytes(winrt::array_view<uint8_t>{
-      reinterpret_cast<uint8_t *>(&script[0]), reinterpret_cast<uint8_t *>(&script[script.length() - 1])});
+      reinterpret_cast<uint8_t *>(&script[0]), reinterpret_cast<uint8_t *>(&script[script.length()])});
   dataReader.Close();
 
   co_return script;