[ffmpeg-qsv][avc_encode] multiprocess avc encode Segmentation fault on iHD driver.

[a]

Summary of the bug:

multiprocess of avc encode cause Segmentation fault

environment

FFMPEG-QSV	CFL	OS: ubuntu 17.04	kernel: 4.15.0
	KBL	OS: ubuntu 16.04	kernel: 4.14.0-rc1
	SKL	OS: ubuntu 16.04	kernel: 4.15.0
			
libva	b6c50dad4d7d14c507108d9f468662e2d7ae1c4d	 https://github.com/01org/libva.git  ( master )
iHD Driver	ed04556a6676fd77ff845508bc54bfc60915450c	 https://github.com/intel/media-driver
libva-utils	9a10ad663349732decd668a426cfc349b0010d1d	 https://github.com/01org/libva-utils.git
Mediasdk	7ea683585f0f84f1bea035a405855312c92c46c0	 https://github.com/Intel-Media-SDK/MediaSDK
FFMPEG	56f68a099cc607658118e00cad30569103ae3751	 https://git.ffmpeg.org/ffmpeg.git

How to reproduce:

1.build env as above lists
2.for(( i=0; i< 10; i++))
do
{ffmpeg -hwaccel qsv -init_hw_device qsv=hw -filter_hw_device hw -v verbose -f rawvideo -video_size 1920x1080 -pix_fmt nv12 -i ./123_217frame_1920x1080_blue_sky.yuv -an -vf hwupload=extra_hw_frames=64,format=qsv -c:v h264_qsv -g 30 -bf 3 -refs 3 -vframes 100 -vsync 0 -y ./H264e_multi_process_1920x1080P_3.h264
}&
done
3. check message

error message

30295 Segmentation fault      (core dumped) 
30296 Segmentation fault      (core dumped) 

yuv file too large to upload