Fix cosmetic keyboard-interactive issue: at the end of a normal k-i session,

some servers (Debian in particular seems prone to this) send a k-i packet with
no prompts and nothing to display. We were printing an extra "Using
keyboard-interactive authentication" message in this case. (Introduced by me
in r8172, I think.)

git-svn-id: svn://svn.tartarus.org/sgt/putty@8492 cda61777-01e9-0310-a592-d414129be87e