avcodec/diracdec: Fix overflow in DC computation

Fixes: runtime error: signed integer overflow: 11896 + 2147483646 cannot be represented in type 'int'
Fixes: 3053/clusterfuzz-testcase-minimized-6355082062856192

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 b5995856a4)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/diracdec.c

@@ -1422,7 +1422,7 @@ static void decode_block_params(DiracContext *s, DiracArith arith[8], DiracBlock
     if (!block->ref) {
         pred_block_dc(block, stride, x, y);
         for (i = 0; i < 3; i++)
-            block->u.dc[i] += dirac_get_arith_int(arith+1+i, CTX_DC_F1, CTX_DC_DATA);
+            block->u.dc[i] += (unsigned)dirac_get_arith_int(arith+1+i, CTX_DC_F1, CTX_DC_DATA);
         return;
     }