avcodec/eac3dec: Fix runtime error: left shift of negative value -3

Fixes: 672/clusterfuzz-testcase-5595018867769344

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
