BUG: Fix bug in parse caused when xml is parsed from string rather than disk.

If xml is parsed from a string its document url could be NULL which causes
an error when building a map of document urls to documents.

core/XdmfCoreReader.cpp

@@ -101,7 +101,7 @@ public:
                          " in XdmfCoreReader::XdmfCoreReaderImpl::parse");
     }
 
-    mDocuments.insert(std::make_pair((char*)mDocument->URL, mDocument));
+    //    mDocuments.insert(std::make_pair((char*)mDocument->URL, mDocument));
     mXPathContext = xmlXPtrNewContext(mDocument, NULL, NULL);
     mXPathMap.clear();
   }