avcodec/truemotion2: Fix overflow in tm2_apply_deltas()

Fixes: signed integer overflow: 1077952576 + 1077952576 cannot be represented in type 'int'
Fixes: 7712/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_TRUEMOTION2_fuzzer-5056281753681920

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/truemotion2.c

@@ -451,7 +451,7 @@ static inline int GET_TOK(TM2Context *ctx,int type)
 /* common operations - add deltas to 4x4 block of luma or 2x2 blocks of chroma */
 static inline void tm2_apply_deltas(TM2Context *ctx, int* Y, int stride, int *deltas, int *last)
 {
-    int ct, d;
+    unsigned ct, d;
     int i, j;
 
     for (j = 0; j < 4; j++){