lavu/fifo: add av_fifo_freep function

Function allows to free fifo and reset freed pointer.

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
lavfi: use av_fifo_freep

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
lavd/jack_audio: use av_fifo_freep

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
ffmpeg: use av_fifo_freep

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
lavf: use av_fifo_freep

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
lavc: use av_fifo_freep

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
Merge remote-tracking branch 'lukaszmluki/master'

* lukaszmluki/master:
  lavc: use av_fifo_freep
  lavf: use av_fifo_freep
  ffmpeg: use av_fifo_freep
  lavd/jack_audio: use av_fifo_freep
  lavfi: use av_fifo_freep
  lavu/fifo: add av_fifo_freep function

Merged-by: Michael Niedermayer <michaelni@gmx.at>