avcodec/truemotion2: Fix integer overflow in tm2_null_res_block()

Fixes: signed integer overflow: 1111638592 - -2122219136 cannot be represented in type 'int'
Fixes: 13441/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_TRUEMOTION2_fuzzer-5732769815068672

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

libavcodec/truemotion2.c

@@ -600,7 +600,8 @@ static inline void tm2_null_res_block(TM2Context *ctx, AVFrame *pic, int bx, int
 {
     int i;
     int ct;
-    int left, right, diff;
+    unsigned left, right;
+    int diff;
     int deltas[16];
     TM2_INIT_POINTERS();