Improve on the bugfix in commit a5abf1e2:

- Reinstate the assert for 'res->ast != NULL'. If it fails, there is a bug
  in the parser, not an error in the input file.
- Provide a distinct error message for the case where p_xref fails on a
  cross-reference stream because of invalid data.
- Only skip storing the invalid section. Try to follow the /Prev entry in
  the stream dictionary to find more sections.