Auto extend workout plot when no workout selected

If no workout is selected (and ERGO/SLOPE mode are also not
selected) then the plot does not extend once you workout
beyond the first hour.

This was due to an uninitialised variable (ergFile) this patch
fixes that.

Fixes #515.

src/ErgFilePlot.cpp

@@ -174,6 +174,7 @@ ErgFilePlot::ErgFilePlot(MainWindow *main) : main(main)
     NowCurve->setYAxis(QwtPlot::yLeft);
 
     bydist = false;
+    ergFile = NULL;
 
     setAutoReplot(false);
 }