Fix for a crash when saving a document after adding a text shape

Summary:
This patch should fix a crash that happens after
KisShapeLayerCanvas::repaint() is called on a cloned image. Actually,
KisShapeLayer::setImage() was implemented incorrectly. It deleted
m_d->converter and m_d->paintDevice, although canvas still owned a link
to them. Therefore the next coming update crashed the application.

BUG:372357

Test Plan:
I could never reproduce the original bug, I only used the results of
Boud's valgrind output. So please test the original bug with the patch
before I push it into 3.1 and impex.

Reviewers: Krita, rempt

Differential Revision: https://phabricator.kde.org/D3452