sps_log2_ctu_size_minus5 is between 0 and 2, with 3 reserved for future
use. The VVC decoder allows sps_log2_ctu_size_minus5 to be 3, and so
ctb_size_y should be at least 16 bits to prevent overflows. An
alternative patch would leave sps_log2_ctu_size_minus5 as 8 bits and
disallow sps_log2_ctu_size_minus5 = 3.
Signed-off-by: Frank Plowman <post@frankplowman.com>
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