Fix incorrect logic in HaveRegisteredOrActiveSnapshot().

Fix incorrect logic in HaveRegisteredOrActiveSnapshot().

This function gave the wrong answer when there's more than one
RegisteredSnapshots entry, whether or not any of them is the
CatalogSnapshot.  This leads to assertion failure in some scenarios
involving fetching toasted data using a cursor.  (As per discussion,
I'm dubious that this is the right contract to be enforcing at all;
but it surely doesn't help to be enforcing it incorrectly.)

Fetching toasted data using a cursor is evidently under-tested,
so add a test case too.

Per report from Erik Rijkers.  This is new code, so no need for
back-patch.

Discussion: https://postgr.es/m/dc9dd229-ed30-6c62-4c41-d733ffff776b@xs4all.nl