XML parser should clear XML error buffer before processing XML data

Both XML parsers use libxml_get_errors() to determine if there were issues the underlying XML library had when processing XML into an object. However, the error buffer is not cleared before the processor loads XML into an object, so any existing errors in the buffer from other Drupal code that executed during the migration of data would create false positives.

This patch fixes it for the SimpleXml parser. I'm not using the more advanced parser anymore, but something similar could be applied there.