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.

source/blender/editors/screen/screen_edit.c

@@ -1195,10 +1195,9 @@ bool ED_screen_set(bContext *C, bScreen *sc)
 		return true;
 	}
 
-	if (ELEM(sc->state, SCREENMAXIMIZED, SCREENFULL)) {
-		/* find associated full */
-		bScreen *sc1;
-		for (sc1 = bmain->screen.first; sc1; sc1 = sc1->id.next) {
+	/* find associated full */
+	for (bScreen *sc1 = bmain->screen.first; sc1; sc1 = sc1->id.next) {
+		if ((sc1 != sc) && ELEM(sc1->state, SCREENMAXIMIZED, SCREENFULL)) {
 			ScrArea *sa = sc1->areabase.first;
 			if (sa->full == sc) {
 				sc = sc1;