pgsql: Make plperl's handling of Perl arrays safer and more consistent.

Make plperl's handling of Perl arrays safer and more consistent.

plperl_func_handler()'s stanza for handling an arrayref result in
a SETOF function could loop forever (or at least till OOM) when
given a tied array, since av_fetch won't necessarily ever return
a null pointer in that case. Be consistent with the other places
where we traverse a perl array: call av_len() once and use len+1
as the loop limit, silently ignoring any null pointers we get back
from that range of subscripts.

But actually, Perl's preferred locution for this seems to be to
use av_count() not av_len()+1. av_count() seems better since
there's less risk of forgetting to add 1. Also, both of those
functions return Size_t (or SSize_t) not int, creating at least
a theoretical overflow hazard. While we're modernizing this,
let's use the correct variable type where we can, and include an
overflow check where we can't.

Reported-by: Claude Code (via Noah Misch)
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Reviewed-by: Andrey Rachitskiy <pl0h0yp1(at)gmail(dot)com>
Discussion: https://postgr.es/m/569769.1786901901@sss.pgh.pa.us
Backpatch-through: 14

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/794473ec222f937db15a4a419000d6278d97421b

Modified Files
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contrib/jsonb_plperl/jsonb_plperl.c | 5 ++---
src/pl/plperl/plperl.c | 41 ++++++++++++++++++++++++++++---------
2 files changed, 33 insertions(+), 13 deletions(-)