Fix malloc/delete mismatch

Summary:
Fixes pointer was allocated with malloc (by libssh) but freed with
delete. This is not safe and causes undefined behavior.

Test Plan: Build and run kio_sftp with address sanitizer

Reviewers: Frameworks, dfaure, sitter, aacid

Reviewed By: aacid

Subscribers: aacid, kde-frameworks-devel, kfm-devel

Tags: Dolphin, Frameworks

Differential Revision: https://phabricator.kde.org/D19767