Remove all the "assert(len>0)" which forbade zero-length writes across the

Remove all the "assert(len>0)" which forbade zero-length writes across the
from_backend() interface, after having made all implementations safe against
being called with len==0 and possibly-NULL/undefined "data".

(This includes making misc.c:bufchain_add() more robust in this area.)

Assertion was originally added 2002-03-01; e.g., see plink.c:1.53 [r1571].

I believe this now shouldn't break anything.

This should hopefully make `ppk-empty-comment' finally GO AWAY. (Tested
with Unix PuTTY.)

git-svn-id: svn://svn.tartarus.org/sgt/putty@3500 cda61777-01e9-0310-a592-d414129be87e