avcodec/bitstream: zero vlc tables on allocation

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/bitstream: fill invalid vlc tables entries as last pass instead of first

This avoids writing entries twice

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/bitstream: try to make vlc init code inherently thread safe

also remove spinlock, it doesnt work on AIX

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