Prevent pooling broken sentinel connections when connecting to slave

We don't know whether sentinel that we connected to works correctly, so we can't put its connection to pool if we got an error

Author: piotrp

lib/resty/redis/connector.lua

@@ -263,12 +263,12 @@ function _M.connect_via_sentinel(self, params)
     else
         -- We want a slave
         local slaves, err = get_slaves(sentnl, master_name)
-        sentnl:set_keepalive()
-
         if not slaves then
             return nil, err
         end
 
+        sentnl:set_keepalive()
+
         -- Put any slaves on 127.0.0.1 at the front
         tbl_sort(slaves, sort_by_localhost)
 
@@ -282,9 +282,9 @@ function _M.connect_via_sentinel(self, params)
         local slave, err, previous_errors = self:try_hosts(slaves)
         if not slave then
             return nil, err, previous_errors
-        else
-            return slave
         end
+
+        return slave
     end
 end