nfsd: fix unlikely NULL deref in mach_creds_match

We really shouldn't allow a client to be created with cl_mach_cred set
unless it also has a principal name.

This also allows us to fail such cases immediately on EXCHANGE_ID as
opposed to waiting and incorrectly returning WRONG_CRED on the following
CREATE_SESSION.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>