Fix WPK interval metric crash

.. recent update to use getWeight() in the ride file
caused a SEGV as interval metric computation is done
on the fly against a 'manufactured' ride file.

.. That ridefile doesn't have a mainwindow reference in
it, so when getWeight() references mainwindow->metricDB
a SEGV occurred.

src/RideSummaryWindow.cpp

@@ -429,6 +429,7 @@ RideSummaryWindow::htmlSummary() const
             bool even = false;
             foreach (RideFileInterval interval, ride->intervals()) {
                 RideFile f(ride->startTime(), ride->recIntSecs());
+                f.mainwindow = mainWindow; // hack, until we refactor athlete and mainwindow
                 for (int i = ride->intervalBegin(interval); i < ride->dataPoints().size(); ++i) {
                     const RideFilePoint *p = ride->dataPoints()[i];
                     if (p->secs >= interval.stop)