Fixed crash due to uninitialized pointer in OpenGL texture glyph cache.

Change-Id: I384f82f1549a81e19566484e12c75ce8df1f314f
Reviewed-on: http://codereview.qt-project.org/6149


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>