from_backend() should always be called with len > 0. Only rlogin

mode ever failed to do this, and only Plink actually had a problem
with it, so this didn't become obvious for a while. rlogin mode is
fixed, and all implementations of from_backend() now contain an
assertion so that we should spot errors of this type more quickly in
future.

git-svn-id: svn://svn.tartarus.org/sgt/putty@1571 cda61777-01e9-0310-a592-d414129be87e