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/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
