[Qt] Crash in ~GraphicsContext3D() when init failed

[Qt] Crash in ~GraphicsContext3D() when init failed
https://bugs.webkit.org/show_bug.cgi?id=82992

Patch by Srikumar Bonda <srikumar.b@gmail.com> on 2012-04-03
Reviewed by Luiz Agostini.

WebKit crashes in ~GraphicsContext3D() while freeing the variables which are
not even allocated because of the init failure in GraphicsContext3D() for Qt.
In addition, Added a safety check in makeContextCurrent() before accessing
GraphicsContext3DPrivate object.

No new tests because the change is about adding null check
before accessing the variables.

* platform/graphics/qt/GraphicsContext3DQt.cpp:
(WebCore::GraphicsContext3D::~GraphicsContext3D):
(WebCore::GraphicsContext3D::makeContextCurrent):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@113123 268f45cc-cd09-0410-ab3c-d52691b4dbfc