BUG: fix an incomplete check in Reader._error_location

It's unclear why `self._obj` can be None, but that's what I'm seeing when trying numpydoc master to build SciPy master. This fix gives the right traceback.

Author: rgommers

numpydoc/docscrape.py

@@ -412,7 +412,7 @@ def _parse(self):
  self[section] = content
 
  def _error_location(self, msg, error=True):
- if hasattr(self, '_obj'):
+ if hasattr(self, '_obj') and self._obj is not None:
  # we know where the docs came from:
  try:
  filename = inspect.getsourcefile(self._obj)