the error should be checked when failed to initialize the dml iterator in pitr

[Leavrth]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. restore

2. What did you expect to see? (Required)

success

3. What did you see instead (Required)

panic because the error is not checked and the iterator is nil.

tidb/br/pkg/task/stream.go

Lines 1269 to 1272 in 71e6696

	updateRewriteRules(rewriteRules, schemasReplace)
	logFilesIter, err := client.LoadDMLFiles(ctx)
	pd := g.StartProgress(ctx, "Restore KV Files", int64(dataFileCount), !cfg.LogProgress)

4. What is your TiDB version? (Required)

v6.5.3