pgsql: Fix unsafe memory management in CloneRowTriggersToPartition().

Fix unsafe memory management in CloneRowTriggersToPartition().

It's not really supported to call systable_getnext() in a different
memory context than systable_beginscan() was called in, and it's
*definitely* not safe to do so and then reset that context between
calls.  I'm not very clear on how this code survived
CLOBBER_CACHE_ALWAYS testing ... but Alexander Lakhin found a case
that would crash it pretty reliably.

Per bug #15828.  Fix, and backpatch to v11 where this code came in.

Discussion: https://postgr.es/m/15828-f6ddd7df4852f473@postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/eaf0292c3ba7acac2735f99a8988bd49a26112f5

Modified Files
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src/backend/commands/tablecmds.c | 14 +++++++-------
1 file changed, 7 insertions(+), 7 deletions(-)