avcodec/truemotion2: Fix 2 integer overflows in tm2_low_res_block()

Fixes: signed integer overflow: 1778647621 + 574372924 cannot be represented in type 'int'
Fixes: 18692/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_TRUEMOTION2_fuzzer-6248679635943424

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

@@ -586,10 +586,10 @@ static inline void tm2_low_res_block(TM2Context *ctx, AVFrame *pic, int bx, int
         last[0] = (int)((unsigned)last[1]  - ctx->D[0] - ctx->D[1] - ctx->D[2] - ctx->D[3])>> 1;
     last[2] = (int)((unsigned)last[1] + last[3]) >> 1;
 
-    t1 = ctx->D[0] + ctx->D[1];
+    t1 = ctx->D[0] + (unsigned)ctx->D[1];
     ctx->D[0] = t1 >> 1;
     ctx->D[1] = t1 - (t1 >> 1);
-    t2 = ctx->D[2] + ctx->D[3];
+    t2 = ctx->D[2] + (unsigned)ctx->D[3];
     ctx->D[2] = t2 >> 1;
     ctx->D[3] = t2 - (t2 >> 1);