Fix resource leak

This is why I do not want auto to be used in Krita's code:

auto bla = createSomething();

doesn't warn that you need to delete bla as much as

Bla *bla = createSomething();

because it's invisible that bla is a pointer.

The alternative is that we use smart pointers _everywhere_, which
isn't feasible...

CCMAIL:kimageshop@kde.org
CID:305464
(cherry picked from commit befdf9b4)