For reasons of sheer brain fade, we (I) was calling systable_endscan()
immediately after systable_getnext() and expecting the tuple returned
by systable_getnext() to still be valid.

That's clearly wrong.  Move the systable_endscan() down below the tuple
usage.

Discovered initially by Pavel Stehule and then also by Alvaro.

Add a regression test based on Alvaro's testing.