pg_backup_archiver.c

warn_or_exit_horribly() was blithely passing a potentially-NULL
string pointer to a %s format specifier.  That works (at least
to the extent of not crashing) on some platforms, but not all,
and since we switched to our own snprintf.c it doesn't work
for us anywhere.

Of the three string fields being handled this way here, I think
that only "owner" is supposed to be nullable ... but considering
that this is error-reporting code, it has very little business
assuming anything, so put in defenses for all three.

Per a crash observed on buildfarm member crake and then
reproduced here.  Because of the portability aspect,
back-patch to all supported versions.