pgsql: Tighten array dimensionality checks in Perl -> SQL array convers

Tighten array dimensionality checks in Perl -> SQL array conversion.

plperl_array_to_datum() wasn't sufficiently careful about checking
that nested lists represent a rectangular array structure; it would
accept inputs such as "[1, []]". This is a bit related to the
PL/Python bug fixed in commit 81eaaf65e, but it doesn't seem to
provide any direct route to a memory stomp. Instead the likely
failure mode is for makeMdArrayResult to be passed fewer Datums than
the claimed array dimensionality requires, possibly leading to a wild
pointer dereference and SIGSEGV.

Per report from Alexander Lakhin. It's been broken for a long
time, so back-patch to all supported branches.

Discussion: https://postgr.es/m/5ebae5e4-d401-fadf-8585-ac3eaf53219c@gmail.com

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/900a8d526ff538d6cd03c3f52ba09fd4dc765915

Modified Files
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src/pl/plperl/expected/plperl_array.out | 43 +++++++++++++++++++++++
src/pl/plperl/plperl.c | 62 +++++++++++++++++++++------------
src/pl/plperl/sql/plperl_array.sql | 37 ++++++++++++++++++++
3 files changed, 119 insertions(+), 23 deletions(-)