[xml/sgml-pkgs] Bug#999900: epubcheck: java.lang.StackOverflowError

[Mathieu Malaterre]

Control: severity -1 important

Since this bug affects a limited set of architecture, I do not believe
severity can be set to `grave` (please correct if wrong).

On amd64:

```
% epubcheck /usr/share/doc/debian-policy/policy.epub
Validating using EPUB version 3.2 rules.
ERROR(PKG-006): /usr/share/doc/debian-policy/policy.epub(-1,-1):
Mimetype file entry is missing or is not the first file in the
archive.
INFO(HTM_053): /usr/share/doc/debian-policy/policy.epub/ch-opersys.xhtml(95,30):
Found an external file link (file://) in file: "reference external"
href="file:///usr/sh".

Check finished with errors
Messages: 0 fatals / 1 error / 0 warnings / 1 info

EPUBCheck completed
```