Fixes #3915 - NoMethodError private method `rhost'

There's no self.rhost, but rhost is defined.

## Verification steps:

You can verify this way:

- [ ] Download SQL Server for testing: http://www.microsoft.com/en-us/server-cloud/products/sql-server/Try.aspx
- [ ] Run mssql_login against your SQL server, make sure you're testing the Windows auth part.

Or you can verify this way (don't need to do both):

- [ ] Do a ```puts rhost``` in method #mssql_login in lib/metasploit/framework/mssql/client.rb
- [ ] Do a ```puts self.rhost``` in the same method.
- [ ] You should trigger the same NoMethodError error for ```self.rhost```. But the other one should print the rhost just fine.