[RenameDialog] Fix crash when renaming single items

Summary:
`m_spinBox` is initialized only when renaming multiple items.

This commit restores the single-item rename logic which was wrongly
removed by commit c5eb4e31.

BUG: 392743
FIXED-IN: 18.03.90

Test Plan: Disable inline renaming and try to rename single or multiple items (and also to undo the jobs).

Subscribers: #dolphin

Differential Revision: https://phabricator.kde.org/D11972