Fix Crash on Interval selection when last ride deleted

If you delete the last ride and then select an interval in the
interval widget Interval summary window crashes. This patch fixes
this. It should have been picked up in the Null ride files tests
from last week, but slipped through.

src/IntervalSummaryWindow.cpp

@@ -34,6 +34,9 @@ IntervalSummaryWindow::~IntervalSummaryWindow() {
 
 void IntervalSummaryWindow::intervalSelected()
 {
+    // if no ride available don't bother
+    if (mainWindow->currentRideItem() == NULL || mainWindow->currentRide() == NULL) return;
+
 	QString html;
     if (mainWindow->allIntervalItems() != NULL) {
         for (int i=0; i<mainWindow->allIntervalItems()->childCount(); i++) {