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
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1545ca9a7463f4ae8451a845774a107ad0757a9f

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