lavc/rawdec: prefer constant AVPALETTE_SIZE over 256*4

Improve readability.

libavcodec/rawdec.c

@@ -171,7 +171,7 @@ static int raw_decode(AVCodecContext *avctx,
     if (context->buffer) {
         int i;
         uint8_t *dst = context->buffer;
-        buf_size = context->length - 256*4;
+        buf_size = context->length - AVPALETTE_SIZE;
         if (avctx->bits_per_coded_sample == 4){
             for(i=0; 2*i+1 < buf_size && i<avpkt->size; i++){
                 dst[2*i+0]= buf[i]>>4;
@@ -194,7 +194,7 @@ static int raw_decode(AVCodecContext *avctx,
        avctx->codec_tag == MKTAG('A', 'V', 'u', 'p'))
         buf += buf_size - context->length;
 
-    len = context->length - (avctx->pix_fmt==PIX_FMT_PAL8 ? 256*4 : 0);
+    len = context->length - (avctx->pix_fmt==PIX_FMT_PAL8 ? AVPALETTE_SIZE : 0);
     if (buf_size < len) {
         av_log(avctx, AV_LOG_ERROR, "Invalid buffer size, packet size %d < expected length %d\n", buf_size, len);
         return AVERROR(EINVAL);