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]

scp.c

@@ -268,6 +268,8 @@ int from_backend(int is_stderr, char *data, int datalen)
     unsigned char *p = (unsigned char *) data;
     unsigned len = (unsigned) datalen;
 
+    assert(len > 0);
+
     /*
      * stderr data is just spouted to local stderr and otherwise
      * ignored.