Fix leaveroom confirm when the room no longer exists

[singiamtel]

When using the setting Confirmation before leaving a room, there's a bug that prevents you from leaving if you have this setting turned on. I haven't found any regression, let me know if you want me to test a specific use case

Example on how to replicate the bug:

• Toggle the setting Confirmation before leaving a room to true
• Create groupchat with /mgc
• Destroy it with /dgc
• Try to leave

[singiamtel]

Also when I try to refresh the site with this setting on I get a browser alert with

This page is asking you to confirm that you want to leave — information you’ve entered may not be saved.

Is this intended?

[mia-pi-git]

Thanks!