pgsql: Fix check_exclusion_or_unique_constraint for UNIQUE NULLS NOT DI

Fix check_exclusion_or_unique_constraint for UNIQUE NULLS NOT DISTINCT.

Adjusting this function was overlooked in commit 94aa7cc5f.  The only
visible symptom (so far) is that INSERT ... ON CONFLICT could go into
an endless loop when inserting a null that has a conflict.

Richard Guo and Tom Lane, per bug #17558 from Andrew Kesper

Discussion: https://postgr.es/m/17558-3f6599ffcf52fd4a@postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d59383924c580a77a2346d9b1284c8589b3d43e2

Modified Files
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src/backend/executor/execIndexing.c       | 18 ++++++++++++------
src/test/regress/expected/constraints.out |  5 +++--
src/test/regress/sql/constraints.sql      |  5 +++--
3 files changed, 18 insertions(+), 10 deletions(-)