PXB-3269 : Fix debug assertion failure on prepare_handle_ren() files.

It is possible that a space_id.ren with content of desired filename,
the destination file name could already exist.

If the source and desitnation to be renamed is same, skip rename.

storage/innobase/xtrabackup/src/ddl_tracker.cc

@@ -879,6 +879,15 @@ bool prepare_handle_ren_files(const datadir_entry_t &entry, void *) {
  return false;
  }
 
+ // space_id.ren is already with the desired name. Nothing to do.
+ if (source_path != nullptr && dest_path != nullptr &&
+ strcmp(source_path, dest_path) == 0) {
+ xb::info() << "prepare_handle_ren_files: ren_file: " << ren_path
+ << " already has desired file name: " << dest_path
+ << " source path is: " << source_path;
+ return true;
+ }
+
  ut_ad(!os_file_exists(dest_path));
 
  xb::info() << "prepare_handle_ren_files: renaming " << fil_space->name