clipboard: Allow a file read to return 0 for EOF

When used with a FreeRDP client on Linux, a file copy operation from
the clipboard detects end-of-file by a read returning 0 bytes. This is
currently marked as an error.

It is assumed that mstsc.exe detects end-of-file in another way, which
is why this has not been found before.

sesman/chansrv/clipboard_file.c

@@ -538,10 +538,12 @@ clipboard_send_file_data(int streamId, int lindex,
  make_stream(s);
  init_stream(s, cbRequested + 64);
  size = g_file_read(fd, s->data + 12, cbRequested);
- if (size < 1)
+ // If we're at end-of-file, 0 is a valid response
+ if (size < 0)
  {
- LOG_DEVEL(LOG_LEVEL_ERROR, "clipboard_send_file_data: read error, want %d got %d",
- cbRequested, size);
+ LOG_DEVEL(LOG_LEVEL_ERROR,
+ "clipboard_send_file_data: read error, want %d got [%s]",
+ cbRequested, g_get_strerror());
  free_stream(s);
  g_file_close(fd);
  clipboard_send_filecontents_response_fail(streamId);