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/native/meta-cursor-renderer-native.c

@@ -924,6 +924,7 @@ is_cursor_hw_state_valid (MetaCursorSprite *cursor_sprite,
     }
 
   g_assert_not_reached ();
+  return FALSE;
 }
 
 #ifdef HAVE_WAYLAND