This reworks the frame skipping code such that the reference
buffers are still updated according to the header.
However it also ensures that the current frame will not end
up in any reference buffer.
Also fixes a hang with frame-multithreading, probably because
get_buffer was already called and would have reset the progress,
however the frame could remain in framep due to the missing update
(or it could be assigned to next_framep and a skip_frame skip would
then write it into framep - there might be even more failure modes).
Sample might become available at samples/nsv/vp8.nsv

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>