The Messenger: fix generation failure for no portal shuffle with 3 available portals

[alwaysintreble]

What is this fixing or adding?

I wrote code to guarantee the searing crags portal is always open when portal shuffle is disabled but i never considered that someone would have their starting portals set to 3 when doing that for some reason.

How was this tested?

Added a test for it that fails without the change.

If this makes graphical changes, please attach screenshots.