Fix DnD crash when source is not present

This resolves Bug 1602
(cherry picked from commit b7d35a6235)

src/kernel/qdnd_x11.cpp

@@ -1214,7 +1214,7 @@ Window findRealWindow( const QPoint & pos, Window w, int md )
 
 void QDragManager::move( const QPoint & globalPos )
 {
-    if (!object) {
+    if (!object || !object->source()) {
         // perhaps the target crashed?
         return;
     }