Fix parsing of XHPAST parse error

Summary: This is the real problem in D5043.

Test Plan:
Linted file with XHP:

  lang=xml
  <a>&
  </a>

Reviewers: epriestley, nh

Reviewed By: epriestley

CC: aran, Korvin, s.o.butler

Differential Revision: https://secure.phabricator.com/D5044