pgsql: Fix unsafe usage of strerror(errno) within ereport().

Fix unsafe usage of strerror(errno) within ereport().

This is the converse of the unsafe-usage-of-%m problem: the reason
ereport/elog provide that format code is mainly to dodge the hazard
of errno getting changed before control reaches functions within the
arguments of the macro. I only found one instance of this hazard,
but it's been there since 9.4 :-(.

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/28782d7e3b923a9eb9e1063c6e3cc69e167fad11

Modified Files
--------------
src/backend/libpq/auth.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)