Fix crash-on-exit in qmlscene

Fix crash-on-exit in qmlscene

Add 0-pointer check to the renderThread access
in maybeUpdate(), which is in some cases called
from ~QSGCanvas() after the thread has been deleted.
(the call happens as a part of SGRootItem destruction.)

Change-Id: Icbcc0d7ad9af54d24ca99f254646b636c2eb9ebb
Reviewed-on: http://codereview.qt.nokia.com/2995
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>