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]

plink.c

@@ -8,6 +8,7 @@
 #include <windows.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <assert.h>
 #include <stdarg.h>
 
 #define PUTTY_DO_GLOBALS	       /* actually _define_ globals */
@@ -147,6 +148,8 @@ int from_backend(int is_stderr, char *data, int len)
     HANDLE h = (is_stderr ? errhandle : outhandle);
     int osize, esize;
 
+    assert(len > 0);
+
     if (is_stderr) {
 	bufchain_add(&stderr_data, data, len);
 	try_output(1);