Ensure that slots are zeroed before use

Ensure that slots are zeroed before use

The previous coding relied on the memory for the slots being zeroed
elsewhere, which while it was true in this case is not an contract
which is guaranteed to hold.  Explicitly clear the tts_isnull array
to ensure that the slots are filled from a known state.

Backpatch to v14 where the catalog multi-inserts were introduced.

Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/CAJ7c6TP0AowkUgNL6zcAK-s5HYsVHVBRWfu69FRubPpfwZGM9A@mail.gmail.com
Backpatch-through: 14