Fix division by zero when split regions are hidden

Code has special handling so that only the first of two split regions
needs to be flagged as hidden. If the second one is also flagged that
can have some unwanted side-effects. Code to add the region resizing
edges relied on the region visibility flags though, rather than the
visibility check that respects this handling of split region hiding. So
code would run that is only intended for un-hidden regions.

source/blender/editors/screen/screen_ops.cc

@@ -782,7 +782,7 @@ static bool azone_clipped_rect_calc(const AZone *az, rcti *r_rect_clip)
   if (az->type == AZONE_REGION) {
     if (region->overlap && (region->v2d.keeptot != V2D_KEEPTOT_STRICT) &&
         /* Only when this isn't hidden (where it's displayed as an button that expands). */
-        ((az->region->flag & (RGN_FLAG_HIDDEN | RGN_FLAG_TOO_SMALL)) == 0))
+        region->visible)
     {
       /* A floating region to be resized, clip by the visible region. */
       switch (az->edge) {