Fix remove message updating top level message lengths when message has a reliability header

[Mukikaizoku]

Noticed issues with reading messages off of a DataReceivedEventArgs object. When removing a message internally, the top most reader will be able to continue reading additional "non-existent" messages off of the MessageReader

Example, assuming evt.Message only contains a top-level message:

MessageReader parentReader = evt.Message;
MessageReader firstMessage = parentReader.ReadMessage();
// Then remove message in a sub message within firstMessage
MessageReader secondMessage = parentReader.ReadMessage();
// secondMessage appears to exist, with default values (0)

During investigation, the issue was traced to the Length of the parent message not being updated during the recursive AdjustLength() calls along the RemoveMessage path

This PR fixes the issue