Fix memory leak in KXzFilter::init

Summary:
I wasn't 99% sure we need to free the filters so early since we're passing them to the functions that manipulate the streams,
but having a look at various other code using lzma they also free them very early and the fact that everything still works is also a good indicator

Test Plan: Before valgrind --leak-check=full karchivetest gave memory leaks, now it does not

Subscribers: apol, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24938