lavfi/gradfun: do not increment DC pointer for odd values.

First DC is only used once otherwise. This also makes the code
consistent with ASM versions.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavfilter/vf_gradfun.c

@@ -56,7 +56,7 @@ DECLARE_ALIGNED(16, static const uint16_t, dither)[8][8] = {
 void ff_gradfun_filter_line_c(uint8_t *dst, uint8_t *src, uint16_t *dc, int width, int thresh, const uint16_t *dithers)
 {
     int x;
-    for (x = 0; x < width; x++, dc += x & 1) {
+    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;