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.)

[originally from svn r3500]
[r1571 == fdbd6978016e9fd87db7b3bfc33ff0da8bd3eea9]