avcodec/dsputil: fix signedness in sizeof() comparissions

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
static void ff_jref_idct1_put(uint8_t *dest, int line_size, int16_t *block)
{
    dest[0] = av_clip_uint8((block[0] + 4)>>3);
}

static void ff_jref_idct1_put(uint8_t *dest, int line_size, int16_t *block)
{
    dest[0] = av_clip_uint8((block[0] + 4)>>3);
}
