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.
