grid: avoid a floating point exception

When a homogeneous grid has no visible children, we were
accidentally doing a division by zero. Instead, just bail
out early in this case, there is nothing to allocate anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=672763

gtk/gtkgrid.c

@@ -1020,6 +1020,9 @@ gtk_grid_request_allocate (GtkGridRequest *request,
 
   gtk_grid_request_compute_expand (request, orientation, &nonempty, &expand);
 
+  if (nonempty == 0)
+    return;
+
   linedata = &priv->linedata[orientation];
   lines = &request->lines[orientation];