[TreePlayer] Fix `TTreeReaderArray::GetSize`
[TreePlayer] Fix `TTreeReaderArray::GetSize`

This is a second part of the fix for ROOT-9570.

root-mirror/root,karies/root,olifre/root,karies/root,olifre/root,root-mirror/root,karies/root,olifre/root,karies/root,root-mirror/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,root-mirror/root,olifre/root,karies/root,olifre/root,olifre/root,olifre/root,karies/root,karies/root,root-mirror/root,karies/root,olifre/root,root-mirror/root,karies/root,root-mirror/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root