save: Report malloc failure in xmlAttrSerializeTxtContent

Flush buffer before checking for errors.

xmlsave.c

@@ -2407,6 +2407,7 @@ xmlAttrSerializeTxtContent(xmlBufferPtr buf, xmlDocPtr doc,
         return;
     out = xmlOutputBufferCreateBuffer(buf, NULL);
     xmlBufAttrSerializeTxtContent(out, doc, string);
+    xmlOutputBufferFlush(out);
     if ((out == NULL) || (out->error))
         xmlFree(xmlBufferDetach(buf));
     xmlOutputBufferClose(out);