libstagefright: fix output buffer is not released by Writer when stopping

  When video recording is stopped due to max file size/duration reached,
  there is a media server crash caused by one output is not released by
  MPEG4Writer. So release the buffer in these cases.

Change-Id: Ib7d15fcf21aea06e856e9adc7b0bcdd01a376328
CRs-Fixed: 782013