pgsql: Fix NULL handling in datum_to_jsonb().

Fix NULL handling in datum_to_jsonb().

The function failed to adhere to its specification that the "tcategory"
argument should not be examined when the input value is NULL.  This
resulted in a crash in some cases.  Per bug #13680 from Boyko Yordanov.

In passing, re-pgindent some recent changes in jsonb.c, and fix a rather
ungrammatical comment.

Diagnosis and patch by Michael Paquier, cosmetic changes by me

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/a93b3782e3358cbb1ad8d65386a2e1478b805649

Modified Files
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src/backend/utils/adt/jsonb.c       |   27 +++++++++++++++------------
src/test/regress/expected/jsonb.out |    9 +++++++++
src/test/regress/sql/jsonb.sql      |    4 ++++
3 files changed, 28 insertions(+), 12 deletions(-)