Do not crash when finishing inline renaming in unusual ways

The crash was caused by a null pointer dereference when, e.g.,
minimizing Dolphin. The root cause was that
KStandardItemListWidget::closeRoleEditor() was called twice: once when
the role editor loses focus, and once again when the window is resized.
After m_roleEditor was set to 0, the second call dereferenced this null
pointer. I think the best solution is to disconnect from the role
editor's signals when the editor is not needed any more by the
KStandardItemListWidget.

CCBUG: 304524
(cherry picked from commit a9c2bdc3)