Fix off-by-one error in Assertion.

The point of the assertion is to ensure that the arrays allocated in stack
are large enough, but the check was one item short.

This won't matter in practice because MaxIndexTuplesPerPage is an
overestimate, so you can't have that many items on a page in reality.
But let's be tidy.

Spotted by Anastasia Lubennikova. Backpatch to all supported versions, like
the patch that added the assertion.