Fix: Possible overwriting of custom asset blend files

The asset editing API wouldn't check if the file it's writing to is
known to be managed by the asset editing API itself, and therefore
writable by the API. So the API could potentially lead to custom blend
files getting overridden.

To be clear, all users of the API had sufficient other checks, so in
practice it wouldn't happen. Still make sure the API is safe to use
without requiring the caller performing extra checks.

source/blender/blenkernel/intern/asset_edit.cc

@@ -267,7 +267,7 @@ std::optional<std::string> asset_edit_id_save_as(Main &global_main,
 
 bool asset_edit_id_save(Main &global_main, const ID &id, ReportList &reports)
 {
-  if (!asset_edit_id_is_editable(id)) {
+  if (!asset_edit_id_is_writable(id)) {
     return false;
   }