[io] prevent nullptr access if oldclass not found

[ferdymercury]

This Pull request:

Changes or fixes:

Fixes https://github.com/root-project/root/issues/8194

Checklist:

• [x] tested changes locally
• [ ] updated the docs (if necessary)

[github-actions[bot]]

Test Results

    13 files      13 suites   2d 22h 48m 19s :stopwatch:  2 648 tests  2 648 :white_check_mark: 0 :zzz: 0 :x: 32 606 runs  32 606 :white_check_mark: 0 :zzz: 0 :x:

Results for commit 337762e9.

:recycle: This comment has been updated with latest results.

[linev]

I will wait until @pcanal will review the PR.

[dpiparo]

Thanks a lot for proposing this solution, appreciated!

[pcanal]

Thanks.