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-month-view.c

@@ -879,6 +879,10 @@ gcal_month_view_drag_drop (GtkWidget      *widget,
 
   cell = get_dnd_cell (widget, x, y);
   event_widget = gtk_drag_get_source_widget (context);
+
+  if (!GCAL_IS_EVENT_WIDGET (event_widget))
+    return FALSE;
+
   event = gcal_event_widget_get_event (GCAL_EVENT_WIDGET (event_widget));
 
   /* Move the event's date */

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;