Fix memory leak in xmlParseBalancedChunkMemoryRecover

When doc is NULL, namespace created in xmlTreeEnsureXMLDecl
is bind to newDoc->oldNs, in this case, set newDoc->oldNs to
NULL and free newDoc will cause a memory leak.

Found with libFuzzer.

Closes #82.

parser.c

@@ -13894,7 +13894,8 @@ xmlParseBalancedChunkMemoryRecover(xmlDocPtr doc, xmlSAXHandlerPtr sax,
     xmlFreeParserCtxt(ctxt);
     newDoc->intSubset = NULL;
     newDoc->extSubset = NULL;
-    newDoc->oldNs = NULL;
+    if(doc != NULL)
+	newDoc->oldNs = NULL;
     xmlFreeDoc(newDoc);
 
     return(ret);