avcodec: remove use of av_init_packet as it is deprecated in new ffmpeg major version
av_init_packet is deprecated in ffmpeg on lavc 58.133.100.
We have good change to update code now, to make change easier than when it is actually removed.
I tested the commits with running valgrind without commits and commits with following commands:
valgrind --leak-check=full vlc -Idummy -vv Videot/sieh_nyx.avi --sout="#transcode{vcodec=mp2v,acodec=mp3,vfilter=deinterlace,width=480,height=300}:file{dst=testi.ts}" --play-and-exit
Output for me seemed to be identical.