Guard against render time query failing

glGetQuery can fail (for example because of a context loss) in this
case the buffer stays unmodified. In this case this is zero resulting
in GLRenderTimeQuery::result() returning a negative value. Down the
line this leads to a negative duration in the RenderJournal and
RenderLoopPrivate::scheduleRepaint starting a timer with an amount
of milliseconds bigger than what an int can hold. This will not
actually start a timer but QTimer::isActive returns true resulting
in no futher repaints being scheduled.
BUG: 475605
FIXED-IN: 6.0


(cherry picked from commit 6b401801)