nbtree: assert no scheduled primscan between pages.

nbtree: assert no scheduled primscan between pages.

Follow-up to bugfix commit 763d65ae.  Technically this new assertion is
redundant with the assertion recently added to _bt_readpage by that same
commit, but it seems like a good idea to have both.

The new assertion makes it clear that we expect to call _bt_readnextpage
when there's another primitive index scan scheduled, though only when
needed as the final step of ending the current primitive scan.