[9.20] fix: dev: Fix inverted gethostname() check in rndc status

The replacement of named_os_gethostname() with raw gethostname()
inverted the success check: the "localhost" fallback runs on success,
and on failure the uninitialized hostname buffer is read by snprintf(),
leaking stack memory via the rndc status reply.

Closes #5889

Backport of MR !11879

Merge branch 'backport-5889-fix-gethostname-inverted-check-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11881