Prevent vrele() being called with a NULL argument if nfs_namei() returns an error in several places.
Prevent vrele() being called with a NULL argument if nfs_namei() returns an
error in several places.

Should fix the "vrele: null vp" panic that henning@ has been seeing lately.

ok tedu@, krw@, deraadt@

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars