avfilter: fix plane validity checks

Fixes out of array accesses

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
void ff_gradfun_filter_line_c(uint8_t *dst, const uint8_t *src, const uint16_t *dc, int width, int thresh, const uint16_t *dithers)
{
    int x;
    for (x = 0; x < width; dc += x & 1, x++) {
        int pix = src[x] << 7;
        int delta = dc[0] - pix;
        int m = abs(delta) * thresh >> 16;
        m = FFMAX(0, 127 - m);
        m = m * m * delta >> 14;
        pix += m + dithers[x & 7];
        dst[x] = av_clip_uint8(pix >> 7);
    }
}

void ff_gradfun_filter_line_c(uint8_t *dst, const uint8_t *src, const uint16_t *dc, int width, int thresh, const uint16_t *dithers)
{
    int x;
    for (x = 0; x < width; dc += x & 1, x++) {
        int pix = src[x] << 7;
        int delta = dc[0] - pix;
        int m = abs(delta) * thresh >> 16;
        m = FFMAX(0, 127 - m);
        m = m * m * delta >> 14;
        pix += m + dithers[x & 7];
        dst[x] = av_clip_uint8(pix >> 7);
    }
}
