clutter: 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

clutter/clutter/clutter-shader-effect.c

@@ -333,6 +333,7 @@ clutter_shader_effect_create_shader (ClutterShaderEffect *self)
 
     default:
       g_assert_not_reached ();
+      return COGL_INVALID_HANDLE;
     }
 }