[#115] rpcclient: Fix type getters

* Add type instance check for nil to avoid panic by
  accessing fields.

Signed-off-by: Airat Arifullin <a.arifullin@yadro.com>

rpc/client/conn.go

@@ -20,5 +20,9 @@ type Conn interface {
 // Client should not be used after Close() call
 // on the connection: behavior is undefined.
 func (c *Client) Conn() io.Closer {
+	if c == nil {
+		return nil
+	}
+
 	return c.conn
 }