* ./src/input/input.c: fixed a few segfaults caused by the dummy plugin.
  * ./plugins/mpeg_adec/mpeg_adec.c: fixed a segfault due to misuse of
    DecoderError.
  * ./plugins/x11/xcommon.c: replaced XFlush with XSync. This must be the
    fourth time someone has to commit this, so I added a reminder :-)