While decoding changes in a loop, if we skip all the changes there is no
CFI making the loop uninterruptible.

Reported-by: Whale Song and Andrey Borodin
Bug: 17580
Author: Masahiko Sawada
Reviwed-by: Amit Kapila
Backpatch-through: 10
Discussion: https://postgr.es/m/17580-849c1d5b6d7eb422@postgresql.org
Discussion: https://postgr.es/m/B319ECD6-9A28-4CDF-A8F4-3591E0BF2369@yandex-team.ru