Fix broken ms12-020 vulnerability detection

The previous version of the script had an off-by-one error that prevented
proper detection of the vulnerability.  Changes made in this revision
include:

 - Correction of the off-by-one error
 - Use of match instead of == to check for valid RDP connection
 - Change of the channel requests to use IDs actually provided by
   the responses from the server
Landing #1857 - MS12-020 off-by-one fix