pgsql: Fix contrib/hstore_plperl to look through scalar refs.

Fix contrib/hstore_plperl to look through scalar refs.

Bring this transform function into sync with the policy established
by commit 3a382983d.

Also, fix it to make sure that what it drills down to is indeed a
hash, and not some other kind of Perl SV. Previously, the test
cases added here provoked crashes.

Because of the crash hazard, back-patch to 9.5 where this module
was introduced.

Discussion: https://postgr.es/m/[email protected]

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e3b7f7cc50630dac958a48b533cce04e4222892b

Modified Files
--------------
contrib/hstore_plperl/expected/hstore_plperl.out | 19 +++++++++++++++++++
contrib/hstore_plperl/hstore_plperl.c | 14 +++++++++++++-
contrib/hstore_plperl/sql/hstore_plperl.sql | 19 +++++++++++++++++++
3 files changed, 51 insertions(+), 1 deletion(-)