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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4e6e375b0039a2afc5830ec5a161536428767e96

Modified Files
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src/backend/access/nbtree/nbtsearch.c | 66 ++++++++++++++++++-----------------
1 file changed, 34 insertions(+), 32 deletions(-)