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

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.

[originally from svn r1571]