avcodec/proresenc_kostya: use unsigned alpha for rotation

Fixes: left shift of negative value -208
Fixes: 69073/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PRORES_KS_fuzzer-4745020002336768

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 93e0265e27)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/proresenc_kostya.c

@@ -342,7 +342,7 @@ static void get_slice_data(ProresContext *ctx, const uint16_t *src,
 
 static void get_alpha_data(ProresContext *ctx, const uint16_t *src,
                            ptrdiff_t linesize, int x, int y, int w, int h,
-                           int16_t *blocks, int mbs_per_slice, int abits)
+                           uint16_t *blocks, int mbs_per_slice, int abits)
 {
     const int slice_width = 16 * mbs_per_slice;
     int i, j, copy_w, copy_h;