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/core/boxes.c

@@ -1246,6 +1246,7 @@ meta_rectangle_edge_aligns (const MetaRectangle *rect, const MetaEdge *edge)
              BOX_LEFT (edge->rect) <= BOX_RIGHT (*rect);
     default:
       g_assert_not_reached ();
+      return FALSE;
     }
 }