Fix BLI_libblock_ensure_unique_name not ignoring linked IDs.

This function would considere that there was a name conflict even in
case existing ID would be a linked one.

This is only a (symbolic) perforance improvement and logical fix, since
`BKE_id_new_name_validate` would not do that mistake anyway.

source/blender/blenkernel/intern/lib_id.c

@@ -2198,7 +2198,7 @@ void BLI_libblock_ensure_unique_name(Main *bmain, const char *name)
 
   /* search for id */
   idtest = BLI_findstring(lb, name + 2, offsetof(ID, name) + 2);
-  if (idtest != NULL) {
+  if (idtest != NULL && !ID_IS_LINKED(idtest)) {
     /* BKE_id_new_name_validate also takes care of sorting. */
     BKE_id_new_name_validate(lb, idtest, NULL, false);
     bmain->is_memfile_undo_written = false;