Does not rely on the pointervalue returned by getNameInDocument() to use as a DAG key.

In order to make getNameInDocument() always return a valid string,
we implement a getDagKey() method that shall be used instead of getNameInDocument()
when we want to use the pointer value as a "key" to identify the DocumentObject.

src/Gui/ViewProviderLink.cpp

@@ -225,7 +225,7 @@ public:
     }
 
     const char *getLinkedName() const {
-        return pcLinked->getObject()->getNameInDocument();
+        return pcLinked->getObject()->getDagKey();
     }
 
     const char *getLinkedLabel() const {