pgsql: jsonb_plperl, jsonb_plpython: Fix unguarded recursion and loops.

jsonb_plperl, jsonb_plpython: Fix unguarded recursion and loops.

Add check_stack_depth() to Jsonb_to_SV, SV_to_JsonbValue,
PLyObject_FromJsonbContainer, and PLyObject_ToJsonbValue. Without
this, deeply nested JSONB values can crash the backend with SIGSEGV
instead of raising a proper error.

Also add CHECK_FOR_INTERRUPTS() to the while loop in SV_to_JsonbValue
that dereferences chains of Perl references, so that a circular
reference (e.g. $x = \$x) can be cancelled by the user instead of
spinning indefinitely. (We looked at detecting such circular
references, but it seems more trouble than it's worth.)

Author: Aleksander Alekseev <aleksander(at)tigerdata(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/CAJ7c6TPbjkzUk4qJ5dHvDNEz0hBuFue3A-XWz_=897z+BC+z8A@mail.gmail.com
Backpatch-through: 14

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a6f23c3ade0effb827255dd9f7df0d9515ce5b8c

Modified Files
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contrib/jsonb_plperl/jsonb_plperl.c | 15 +++++++++++++++
contrib/jsonb_plpython/jsonb_plpython.c | 7 +++++++
2 files changed, 22 insertions(+)