From H.266 (V3) (09/2023) p. 321:

It is a requirement of bitstream conformance that the luma block
vector bvL shall obey the following constraints:
- CtbSizeY is greater than or equal to
((yCb + (bvL[ 1 ] >> 4)) & (CtbSizeY − 1)) + cbHeight

This patch checks this is true, which fixes crashes on fuzzed
bitstreams.
Signed-off-by: Frank Plowman <post@frankplowman.com>