This fixes frame corruption issue when decoder started reusing frames
while they are still in use of encoding process

Issue with frame corruption  was reproduced using:

    avconv.exe -y -hwaccel d3d11va -hwaccel_output_format d3d11 -i input.h264 -an -c:v h264_amf output.mkv

It is recommended to use -extra_hw_frames 16 option in case if hw frames
number in pool is not enough
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>