avcodec/truemotion2: Fix integer overflow in last loop in tm2_update_block()

Fixes: signed integer overflow: -1727985666 - 538976288 cannot be represented in type 'int'
Fixes: 15031/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_TRUEMOTION2_fuzzer-5100228035739648

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 3aecd0170413c7e56f19de4e34d093a2c4027c2a)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/truemotion2.c

@@ -672,7 +672,7 @@ static inline void tm2_still_block(TM2Context *ctx, AVFrame *pic, int bx, int by
 static inline void tm2_update_block(TM2Context *ctx, AVFrame *pic, int bx, int by)
 {
     int i, j;
-    int d;
+    unsigned d;
     TM2_INIT_POINTERS_2();
 
     /* update chroma */