avcodec/diracdec: Fix integer overflow in signed multiplication in UNPACK_ARITH()

Fixes: runtime error: signed integer overflow: 1073741823 * 4 cannot be represented in type 'int'
Fixes: 2729/clusterfuzz-testcase-minimized-5902915464069120

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

libavcodec/diracdec.c

@@ -454,7 +454,8 @@ static inline int coeff_unpack_golomb(GetBitContext *gb, int qfactor, int qoffse
     static inline void coeff_unpack_arith_##n(DiracArith *c, int qfactor, int qoffset, \
                                               SubBand *b, type *buf, int x, int y) \
     { \
-        int coeff, sign, sign_pred = 0, pred_ctx = CTX_ZPZN_F1; \
+        int sign, sign_pred = 0, pred_ctx = CTX_ZPZN_F1; \
+        unsigned coeff; \
         const int mstride = -(b->stride >> (1+b->pshift)); \
         if (b->parent) { \
             const type *pbuf = (type *)b->parent->ibuf; \