pgsql: Fix IsValidJsonNumber() to notice trailing non-alphanumeric garb

Fix IsValidJsonNumber() to notice trailing non-alphanumeric garbage.

Commit e09996ff8dee3f70 was one brick shy of a load: it didn't insist
that the detected JSON number be the whole of the supplied string.
This allowed inputs such as "2016-01-01" to be misdetected as valid JSON
numbers. Per bug #13906 from Dmitry Ryabov.

In passing, be more wary of zero-length input (I'm not sure this can
happen given current callers, but better safe than sorry), and do some
minor cosmetic cleanup.

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/1c291624b29dae2049ff1c7008563fbca85df658

Modified Files
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contrib/hstore/expected/hstore.out | 16 ++++++------
contrib/hstore/sql/hstore.sql | 4 +--
src/backend/utils/adt/json.c | 52 ++++++++++++++++++++++++--------------
3 files changed, 43 insertions(+), 29 deletions(-)