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 :-(.