[CoordinatedGraphics] Segmentation fault at  CoordinatedGraphicsScene::clearImageBackingContents

https://bugs.webkit.org/show_bug.cgi?id=125776

Patch by Edit Balint <edbalint@inf.u-szeged.hu> on 2014-01-20
Reviewed by Csaba Osztrogonác.

Unexpected behavior occurs in some test cases which leads to segmentation fault.

* platform/graphics/texmap/coordinated/CompositingCoordinator.cpp:
(WebCore::CompositingCoordinator::removeImageBacking):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@162329 268f45cc-cd09-0410-ab3c-d52691b4dbfc
Task-number: QTBUG-38841
Change-Id: I85699c26f9ec78d97d66cf734188ba7da7e2233e
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>