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>

fs/nfs/client.c

@@ -314,7 +314,8 @@ static struct nfs_client *nfs_match_client(const struct nfs_client_initdata *dat
 		/* Match the full socket address */
 		if (!rpc_cmp_addr_port(sap, clap))
 			/* Match all xprt_switch full socket addresses */
-			if (!rpc_clnt_xprt_switch_has_addr(clp->cl_rpcclient,
+			if (IS_ERR(clp->cl_rpcclient) ||
+                            !rpc_clnt_xprt_switch_has_addr(clp->cl_rpcclient,
 							   sap))
 				continue;