Fix random bug when workstation_name is < 6 chars

When the local workstation name is less than 6 characters, remote
authentication against a Windows 2008r2 WinRM service always fails. This
doesn't seem to affect authentication against IIS's negotiate
implementation.

VERIFICATION
On a target system:
 - [x] Set up winrm server as local admin
     * `winrm quickconfig`
     * `winrm set winrm/config/service @{AllowUnencrypted="true"}`
 - [x] Add msfadmin if you don't already have one `net user /add msfadmin msfadmin` 
 - [x] Add msfadmin to local admins `net localgroup administrators /add msfadmin`

On attack platform:
 - [x] launch msfconsole
 - [x] `use auxiliary/scanner/winrm/winrm_login`
 - [x] set RHOST to the address of the Windows box you set up with the above
 - [x] `set USERNAME msfadmin`
 - [x] `set PASSWORD msfadmin`
 - [x] `run` a bunch of times
 - [x] VERIFY: **before** - intermittent failures with valid credentials; **after** - success every time