fix(oauth): validate that redirect param exists.

Fixes #1786 

@zaach @ckarlof r?
In the [validate](https://github.com/mozilla/fxa-content-server/blob/cdc16f433157b37fc254e7dc6d33b5f4f6b43cf4/app/scripts/lib/validate.js) module, we have `isCodeValid` - see [this snippet](https://github.com/mozilla/fxa-content-server/blob/cdc16f433157b37fc254e7dc6d33b5f4f6b43cf4/app/scripts/lib/validate.js#L50).