views: protect against invalid drop

If the user tries to drop a e.g. file, GNOME Calendar crashes
immediately.

While the desired behavior would be checking if that's an ics file,
lets just not crash for now.

https://bugzilla.gnome.org/show_bug.cgi?id=771680

src/gcal-year-view.c

@@ -1342,6 +1342,9 @@ navigator_drag_drop_cb (GcalYearView   *self,
 
   if (calculate_day_month_for_coord (self, x, y, &day, &month, &is_title))
     {
+      if (!GCAL_IS_EVENT_WIDGET (gtk_drag_get_source_widget (context)))
+        return FALSE;
+
       if (!is_title)
         {
           GcalEventWidget *event_widget;