Calculate the smallest buffer size needed

[clarkli86]

Hi @securesocketfunneling ,

Thanks a lot for making this amazing library for C++ users. The interrogation with boost asio makes it so easy to be to adopted into existing projects.

While testing udt_client with appserver from the original udt library (http://udt.sourceforge.net/), buffer overflow was reported when the packets arrive out of sequence. This commit is to fix that.