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 <pg@bowt.ie>
Reviewed-By: Masahiro Ikeda <ikedamsh@oss.nttdata.com>
Discussion: https://postgr.es/m/CAH2-WznFkEs9K1PtNruti5JjawY-dwj+gkaEh_k1ZE+1xLLGkA@mail.gmail.com