qcam: Fix unregister of not registered timer

If an unregistered timer is given to QtEventDispatcher::unregisterTimer()
an error is printed and Qt gets confused and locks up. Fix this by
following the libcamera documentation that passing a unregistered timer
is a no-op.

Signed-off-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/qcam/qt_event_dispatcher.cpp

@@ -120,6 +120,9 @@ void QtEventDispatcher::registerTimer(Timer *timer)
 void QtEventDispatcher::unregisterTimer(Timer *timer)
 {
 	auto it = timerIds_.find(timer);
+	if (it == timerIds_.end())
+		return;
+
 	timers_.erase(it->second);
 	killTimer(it->second);
 	timerIds_.erase(it);