nfsd4: Fix MACH_CRED NULL dereference

Fixes a NULL-dereference on attempts to use MACH_CRED protection over
auth_sys.
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4state.c

@@ -1264,6 +1264,8 @@ static bool svc_rqst_integrity_protected(struct svc_rqst *rqstp)
 	struct svc_cred *cr = &rqstp->rq_cred;
 	u32 service;
 
+	if (!cr->cr_gss_mech)
+		return false;
 	service = gss_pseudoflavor_to_service(cr->cr_gss_mech, cr->cr_flavor);
 	return service == RPC_GSS_SVC_INTEGRITY ||
 	       service == RPC_GSS_SVC_PRIVACY;