Fix DnD crash when source is not present
This resolves Bug 1602

diff --git a/src/kernel/qdnd_x11.cpp b/src/kernel/qdnd_x11.cpp
index be72799..de549ff 100644
--- a/src/kernel/qdnd_x11.cpp
+++ b/src/kernel/qdnd_x11.cpp
@@ -1214,7 +1214,7 @@
 
 void QDragManager::move( const QPoint & globalPos )
 {
-    if (!object) {
+    if (!object || !object->source()) {
         // perhaps the target crashed?
         return;
     }