Prevent hang during loading of xlsm doc.

* We need to call xmlInitParser() before using it in a multi-threaded
  way (according to the libxml2 doc).
* Better to use 'rEntity' which references the one stored in the vector
  rather than 'entity' whose copy gets stored in the vector. On Windows
  at least 'rEntity' and 'entity' hold different parser pointer values.
* Free the parser before popping the entity. Popping the entity first then
  freeing the parser using its pointer value stored in the entity looks
  weird.

Change-Id: I6b64a6d8ac9c1d4fea8339d8fb2d38dfffbba47b