Ensure node exists before being redirected via an ASK

[jpallen]

If a master node in the Redis cluster does not have any slots, it is not returned by the cluster slots command, and so does not have a client in the connectionPool. If slots are migrated to this new master node, the old master node returns an ASK error with the new node details. At the moment ioredis looks up the node in the connectionPool but it doesn't exist, so the following error is returned:

TypeError: Cannot call method 'asking' of undefined
  at tryConnection (/var/www/document-updater/releases/5543/node_modules/ioredis/lib/cluster/index.js:464:19)
  at Object._this.handleError.ask (/var/www/document-updater/releases/5543/node_modules/ioredis/lib/cluster/index.js:404:11)
  at Cluster.handleError (/var/www/document-updater/releases/5543/node_modules/ioredis/lib/cluster/index.js:503:52)
  at Command.command.reject (/var/www/document-updater/releases/5543/node_modules/ioredis/lib/cluster/index.js:389:13)
  at Redis.exports.returnError (/var/www/document-updater/releases/5543/node_modules/ioredis/lib/redis/parser.js:75:18)

It looks like a similar issues with a MOVED reply was fixed in 0dcb768. This patch applies the same fix to the ASK error.

[luin]

Released in v2.2.0. Thank you for the patch! 🍻

[jpallen]

Wow, that was super quick! I see you've already done another release too, thanks so much :)