Fixes small memory leak in DolphinContextMenu

Summary: When DolphinContextMenu::baseFileItem is called a KFileItem is allocated and stored in DolphinContextMenu::m_baseFileItem, which is of type KFileItem*, but the destructor failed to delete this.

Test Plan: This leak was found using Address Sanitizer

Reviewers: broulik, elvisangelaccio

Reviewed By: broulik, elvisangelaccio

Subscribers: ngraham, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D14178