avcodec/mpegvideoenc: fix undefined negative left shift

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

I can't reproduce the runtime behavior reported in the ticket, hence I
can't confirm that this actually fixes the exact issue reported in the
ticket.

Regardless, I can confirm that this is a genuine issue, and that
negative shifts can (and do) occur, fixed by this.

Tested with FATE.

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