Don't call positionMapTimer.elapsed() until timer is running.

We actually ignore the value returned until positionMapTimer.start() has
been called, so this won't cause any program misbehaviour. But we might
as well use the timer API as intended.

mythtv/libs/libmythtv/recorders/recorderbase.cpp

@@ -472,7 +472,8 @@ void RecorderBase::SavePositionMap(bool force)
     positionMapLock.lock();
 
     uint delta_size = positionMapDelta.size();
-    uint pm_elapsed = positionMapTimer.elapsed();
+    uint pm_elapsed = (positionMapTimer.isRunning()) ?
+        positionMapTimer.elapsed() : 0;
     // save on every 1.5 seconds if in the first few frames of a recording
     needToSave |= (positionMap.size() < 30) &&
         (delta_size >= 1) && (pm_elapsed >= 1500);