Blender Git Loki

Fix crash activating a fullscreened screen

Steps to reproduce were:
* Duplicate some area into new window (shift-click corner triangle)
* Make it fullscreen
* Close the window again
* Activate the added screen from the menu (the one without the
-nonnormal prefix)
-> Crash (you may have to press "Back to Previous" first though)

When activating a screen, code should check if there's a fullscreen
variant of it and activate that instead. From what I can tell that's
what the code tried to do, but incorrectly.

Same issue as T64045, but things are a bit different for 2.7.