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/App/Link.cpp

@@ -468,7 +468,7 @@ void LinkBaseExtension::setOnChangeCopyObject(
         }
     }
 
-    const char *key = flags.testFlag(OnChangeCopyOptions::ApplyAll) ? "*" : parent->getNameInDocument();
+    const char *key = flags.testFlag(OnChangeCopyOptions::ApplyAll) ? "*" : parent->getDagKey();
     if (external)
         prop->setValue(key, exclude ? "" : "+");
     else