pgsql: Avoid possible longjmp-induced logic error in PLy_trigger_build_

Avoid possible longjmp-induced logic error in PLy_trigger_build_args.

The "pltargs" variable wasn't marked volatile, which makes it unsafe
to change its value within the PG_TRY block. It looks like the worst
outcome would be to fail to release a refcount on Py_None during an
(improbable) error exit, which would likely go unnoticed in the field.
Still, it's a bug. A one-liner fix could be to mark pltargs volatile,
but on the whole it seems cleaner to arrange things so that we don't
change its value within PG_TRY.

Per report from Xing Guo. This has been there for quite awhile,
so back-patch to all supported branches.

Discussion: https://postgr.es/m/CACpMh+DLrk=fDv07MNpBT4J413fDAm+gmMXgi8cjPONE+jvzuw@mail.gmail.com

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d96f72569743842eb3d0cf4e97a77427b2729f38

Modified Files
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src/pl/plpython/plpy_exec.c | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)