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]