We cannot drop the reference count to zero while having
no lock. Otherwise, another thread might race doing
s = nm_ref_string_new("...");
nm_ref_string_unref(s);
and already successfully delete the instance.
Hitting this race should be rather difficult, especially because
we tend to use NMRefString only from one thread. But still, access
to global variables must be race free.
Fixes: 908fadec96 ('shared: add NMRefString')
(cherry picked from commit 3490a09a7d)
(cherry picked from commit 1a68a54471)
(cherry picked from commit 71f56a5d76)
ffd87f7718