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