Add braces for if block with large comment in psql's common.c
A patch touching this area of the code is under review, and this format
makes the readability of the code slightly harder to parse.
Extracted from a larger patch by the same author.
Author: Anthonin Bonnefoy <anthonin.bonnefoy@datadoghq.com>
Discussion: https://postgr.es/m/CAO6_XqroE7JuMEm1sWz55rp9fAYX2JwmcP_3m_v51vnOFdsLiQ@mail.gmail.com