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
 - [ ] Set up winrm server (`winrm quickconfig` as local admin)
 - [ ] Add msfadmin if you don't already have one `net user /add msfadmin msfadmin` 
 - [ ] Add msfadmin to local admins `net localgroup administrators /add msfadmin`
 - [ ] launch msfconsole
 - [ ] `use auxiliary/scanner/winrm/winrm_login`
 - [ ] `set USERNAME msfadmin`
 - [ ] `set PASSWORD msfadmin`
 - [ ] `run`
 - [ ] VERIFY: **before** - intermittent failures with valid credentials; **after** - success every time