parser: Fix error return of xmlParseBalancedChunkMemory

Only return an error code if the chunk is not well-formed to match the
2.12 behavior. Return 0 on non-fatal errors like invalid namespaces.

Fixes #765.

Author: nwellnhof

parser.c

@@ -12535,7 +12535,10 @@ xmlParseBalancedChunkMemoryRecover(xmlDocPtr doc, xmlSAXHandlerPtr sax,
     else
         xmlFreeNodeList(list);
 
-    ret = ctxt->errNo;
+    if (!ctxt->wellFormed)
+        ret = ctxt->errNo;
+    else
+        ret = XML_ERR_OK;
 
     xmlFreeInputStream(input);
     xmlFreeParserCtxt(ctxt);