vdpau: avoid use after freed in vlVdpVideoMixerUpdateDeinterlaceFilter

If you gonna check part of code above, you're gonna see that part of making pointer nil is missing:

   /* remove existing filter */
   if (vmixer->deint.filter) {
      vl_deint_filter_cleanup(vmixer->deint.filter);
      FREE(vmixer->deint.filter);
      vmixer->deint.filter = NULL;
   }