avcodec/put_bits: Don't set size_in_bits, fix overflow

avcodec/put_bits: Don't set size_in_bits, fix overflow

A PutBitContext has a field called size_in_bits which is set to the
context's bitsize init_put_bits(); but it isn't used at all (the PutBits
API uses pointers directly and not bit indexes), so remove it (due to
ABI concerns the actual element is only removed at the next bump).

Furthermore, the multiplication inherent in setting this field can lead
to undefined integer overflows. This is particularly true for FFV1,
which uses a very big worst-case buffer (37*4*width*height; even
ordinary 1080p triggers an overflow). Ticket #8350 is about this
overflow which this commit fixes.

This means that the effective range of the PutBits API is no longer
restricted by the /8 as long as one isn't using put_bits_(count|left).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>