avcodec/ac3enc: fix undefined negative left shift

This should fix the undefined behavior reported in:
https://trac.ffmpeg.org/ticket/4727.

I can reproduce this at runtime: simply stick in an abort call in
asym_quant to check if c < 0 and run FATE. I don't know ac3 so I can't
confirm if negative coefficients are intentional, but at the moment they
clearly are according to FATE.

This resolves the undefined behavior. Tested with FATE.

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
