avcodec/cavsdsp: use av_clip_uint8() for idct

      Fixes out of array read
Fixes: 1.swf

Found-by: 连一汉 <lianyihan@360.cn>
Tested-by: 连一汉 <lianyihan@360.cn>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>