rdma: Check that port index exists before operate on link layer

Link layer operates on port layer, hence it should check
it existence before execution commands.

Fixes: da990ab40a ("rdma: Add link object")
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: David Ahern <dsahern@gmail.com>

rdma/link.c

@@ -277,6 +277,9 @@ static int link_one_show(struct rd *rd)
 		{ 0 }
 	};
 
+	if (!rd->port_idx)
+		return 0;
+
 	return rd_exec_cmd(rd, cmds, "parameter");
 }