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 == fdbd697801]

misc.c

@@ -181,6 +181,8 @@ void bufchain_add(bufchain *ch, const void *data, int len)
 {
     const char *buf = (const char *)data;
 
+    if (len == 0) return;
+
     ch->buffersize += len;
 
     if (ch->tail && ch->tail->buflen < BUFFER_GRANULE) {