The number of bits in a PutBitContext must fit into an int, yet nothing
guaranteed the size argument cbs_write_unit_data() uses in init_put_bits()
to be in the range 0..INT_MAX / 8. This has been changed.

Furthermore, the check 8 * data_size > data_bit_start that there is
data beyond the initial padding when writing mpeg2 or H.264/5 slices
could also overflow, so divide it by 8 to get an equivalent check
without this problem.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>