Fix builds with G_DISABLE_ASSERT

Commit 25f416c13d added additional compilation warnings, including
-Werror=return-type. There are several places where this results
in build failures if `g_assert_not_reached()` is disabled at compile
time and the compiler misses a return value.

https://gitlab.gnome.org/GNOME/mutter/issues/447

src/backends/x11/meta-monitor-manager-xrandr.c

@@ -173,6 +173,7 @@ meta_monitor_transform_to_xrandr (MetaMonitorTransform transform)
     }
 
   g_assert_not_reached ();
+  return 0;
 }
 
 static gboolean

src/backends/x11/meta-renderer-x11.c

@@ -70,6 +70,7 @@ get_x11_cogl_winsys_vtable (CoglRenderer *renderer)
       break;
     }
   g_assert_not_reached ();
+  return NULL;
 }
 
 static CoglRenderer *