avcodec/bitstream: document the double volatile

Suggested-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/libtwolame: fix encoding lsf with defaults

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