fix #3038: check user before renewing ticket

fix #3038: check user before renewing ticket

Fixes a bug in which the userid of the ticket cache is updated,
when a user connects, but the ticket itself is not.
This means a newly connected user has a previously connected
user's ticket and thus, cannot do anything, as the client will
attempt to use the invalid ticket.

e.g. if john@pbs connected to the server first, followed by
mike@pbs, the following would be stored in the ticket cache.

{
  "localhost": {
    "mike@pbs": {
      "ticket": "PBS:john@pbs:AAAA",
      "timestamp": 1601039326,
      "token": "BBBB"
    }
  }
}

Signed-off-by: Dylan Whyte <d.whyte@proxmox.com>