Fix T55298: Failing assert when splitting areas vertically

There are more related glitches for which I have fixes. Will test them some more
and push in a followup commit.

source/blender/editors/screen/screen_edit.c

@@ -216,7 +216,7 @@ static short testsplitpoint(ScrArea *sa, char dir, float fac)
 {
 	short x, y;
 	const short area_min_x = AREAMINX;
-	const short area_min_y = ED_area_headersize();
+	const short area_min_y = ED_area_headersize() + 1;
 
 	// area big enough?
 	if (dir == 'v' && (sa->v4->vec.x - sa->v1->vec.x <= 2 * area_min_x)) return 0;