h264: remove clear_blocks call in threading init.

Init code in that if statement goes down from 26716 cycles to 26047
cycles, i.e. the removal of the clear_blocks and smaller memcpy()
together save around 670 cycles.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
h264: Use mb itself as memcpy anchor and assert the other anchors position

This makes the code more robust against reordering or fields.

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