fixed libxml internal buffer leak

[nerg4l]

libxml_use_internal_errors(true) starts to write errors to an internal buffer. That buffer should be cleared after usage.

The documentation of libxml_use_internal_errors says:

[...] Disabling will also clear any existing libxml errors.

But this does not seem to be true. However, the top "User Contributed Notes" says the following:

When using this funtion, be sure to clear your internal error buffer. If you dn't and you are using this in a long running process, you may find that all your memory is used up.

The solution is to call libxml_clear_errors after using the internal error buffer.

[oscarotero]

thank you!