remote an unused buffer in mm_login() (issue #8038, selsky)

In passing, I have cleaned up some formatting to better comply with our
guidelines.  I have also changed one place to use S_OR(), and a couple of
places to use ast_strlen_zero() as appropriate.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@43754 65c4cc65-6c06-0410-ace0-fbb531ad65f3