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]

terminal.c

@@ -3603,6 +3603,8 @@ int term_ldisc(int option)
  */
 int from_backend(int is_stderr, char *data, int len)
 {
+    assert(len > 0);
+
     bufchain_add(&inbuf, data, len);
 
     /*