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/wayland/meta-wayland-xdg-shell.c

@@ -1832,6 +1832,7 @@ positioner_anchor_to_placement_anchor (uint32_t anchor)
       return (META_PLACEMENT_ANCHOR_BOTTOM | META_PLACEMENT_ANCHOR_RIGHT);
     default:
       g_assert_not_reached ();
+      return META_PLACEMENT_ANCHOR_NONE;
     }
 }
 
@@ -1860,6 +1861,7 @@ positioner_gravity_to_placement_gravity (uint32_t gravity)
       return (META_PLACEMENT_GRAVITY_BOTTOM | META_PLACEMENT_GRAVITY_RIGHT);
     default:
       g_assert_not_reached ();
+      return META_PLACEMENT_GRAVITY_NONE;
     }
 }