[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 c5eb4e31161ccf422.
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