fix drag/drop crash regression (230753). r+sr=dbaron.

widget/src/gtk/nsWindow.cpp

@@ -3395,6 +3395,7 @@ nsWindow::OnDragDropSignal        (GtkWidget        *aWidget,
   InitDragEvent(event);
 
   event.message = NS_DRAGDROP_DROP;
+  event.widget = innerMostWidget;
   event.point.x = retx;
   event.point.y = rety;
 

widget/src/gtk2/nsWindow.cpp

@@ -1901,6 +1901,7 @@ nsWindow::OnDragDropEvent(GtkWidget *aWidget,
     innerMostWidget->DispatchEvent(&event, status);
 
     event.message = NS_DRAGDROP_DROP;
+    event.widget = innerMostWidget;
     event.point.x = retx;
     event.point.y = rety;