Noticed via -fsanitize=undefined. Introduced when a few columns in
GetConfigOptionByNum() / pg_settings started to be translated in 72be8c29 /
PG 12.

Backpatch to all affected branches, for the same reasons as 46ab07ffda9.

Discussion: https://postgr.es/m/20220323173537.ll7klrglnp4gn2um@alap3.anarazel.de
Backpatch: 12-