Prevent VOTable validator from crashing when xmllint encounters SIGKILL

This happened when 'vo.xml' was too large (400+ MB), causing memory problem and resulted in system sending kill signal to the process. 

The offending URL was a Cone Search query http://vo.astronet.ru/sai_cas/conesearch?cat=sdssdr7&tab=photoobjall&SR=1&DEC=0&RA=0&VERB=3

    >>> from astropy.io.votable.validator.result import Result
    >>> r = Result(url)
    >>> r.validate_vo()

This fix was suggested by @mdboom and would just set `xmllint` result as `None` instead of crashing. However, it does not fix the memory issue, it just catches the resultant error.

    >>> r._attributes
    {u'network_error': None, ...,
     u'xmllint': None,
     u'xmllint_content': "xmllint was terminated by signal 'SIGKILL'"}

There are no change log entry nor test for `Result` because it is not documented and not meant to be an end-user facing API.