avcodec/flacenc: Fix Invalid Rice order

Fixes ticket #4628.

The problem arose, in the sample file at least, in the last block where the
minimum and maximum Rice partition orders were both 0. In that case, and any
other where pmax == pmin, the original UINT32_MAX placeholder value for
bits[opt_porder] was getting overwritten before the comparison to check if the
current partition order is a new optimal, so the correct partition order and
RiceContext params were not being set.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
