NFS: Fix error handling

Fix an error handling problem: nfs_put_client() can be given a NULL pointer if
nfs_free_server() is asked to destroy a partially initialised record.
Signed-Off-By: David Howells <dhowells@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/client.c

@@ -208,6 +208,9 @@ static void nfs_free_client(struct nfs_client *clp)
  */
 void nfs_put_client(struct nfs_client *clp)
 {
+	if (!clp)
+		return;
+
 	dprintk("--> nfs_put_client({%d})\n", atomic_read(&clp->cl_count));
 
 	if (atomic_dec_and_lock(&clp->cl_count, &nfs_client_lock)) {