If we don't copy this value first, it is seen as 0 by h264_slice_header_init,
due to zero-allocation of the new context, triggering an old hack that
multiplied the denominator by 2 for files produced by old x264 versions, but
only if more than one thread was used.

Fixes #7475 and #7083.
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>