Add nbtree amgettuple return item function.

Add nbtree amgettuple return item function.

This makes it easier to add precondition assertions.  We now assert that
the last call to _bt_readpage succeeded, and that the current item index
is within the bounds of the currPos items array.

Author: Peter Geoghegan <[email protected]>
Reviewed-By: Masahiro Ikeda <[email protected]>
Discussion: https://postgr.es/m/CAH2-WznFkEs9K1PtNruti5JjawY-dwj+gkaEh_k1ZE+1xLLGkA@mail.gmail.com