NFS: Ignore connections that have cl_rpcclient uninitialized

cl_rpcclient starts as ERR_PTR(-EINVAL), and connections like that
are floating freely through the system.  Most places check whether
pointer is valid before dereferencing it, but newly added code
in nfs_match_client does not.

Which causes crashes when more than one NFS mount point is present.

Signed-off-by: Petr Vandrovec <petr@vandrovec.name>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>