Fix various unused variable warnings

ffmpeg.c

ffplay.c

libavcodec/a64multienc.c

libavcodec/celp_filters.c

libavcodec/dca.c

libavcodec/dirac.c

libavcodec/rv10.c

libavcodec/s302m.c

libavcodec/shorten.c

libavcodec/sp5xdec.c

libavcodec/truemotion2.c

libavcodec/vaapi_mpeg2.c

libavcodec/vmdav.c

libavcodec/zmbvenc.c

libavfilter/vf_fieldorder.c

libavformat/4xm.c

libavformat/apetag.c

libavformat/asfdec.c

libavformat/mmf.c

libavformat/mov.c