monitor: Allow %NULL mode in calculate_mode_scale()

If no global scaling factor has been set, we currently segfault when
called with a %NULL mode parameter, which happens when calculating the
scale for the current mode while no modes are available.

Adding %NULL checks in the corresponding callers would mean that we'd
have to duplicate the global scaling factor handling, so instead consider
%NULL mode parameters valid and handle them gracefully.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/41

src/backends/meta-monitor.c

@@ -1458,6 +1458,9 @@ calculate_scale (MetaMonitor     *monitor,
 
   scale = 1.0;
 
+  if (monitor_mode == NULL)
+    goto out;
+
   meta_monitor_mode_get_resolution (monitor_mode,
                                     &resolution_width,
                                     &resolution_height);