This appears to be an issue with Word2000 handling of empty attributes.
A reproduction case can be seen here:
```
$ cat test.html
<html xmlns:o="urn:schemas-microsoft-com:office:office">
<body>
<table>
<img class="" />
</table>
</body>
</html>
$ ./tidy --tidy-mark no --word-2000 yes test.html
line 1 column 1 - Warning: missing <!DOCTYPE> declaration
line 3 column 1 - Warning: <img> isn't allowed in <table> elements
line 2 column 1 - Info: <table> previously mentioned
line 1 column 57 - Warning: inserting missing 'title' element
line 3 column 1 - Warning: <img> lacks "alt" attribute
line 3 column 1 - Warning: <img> lacks "src" attribute
line 2 column 1 - Warning: trimming empty <table>
line 1 column 1 - Warning: <html> proprietary attribute "xmlns:o"
[2] 52405 segmentation fault ./tidy --tidy-mark no --word-2000 yes test.html
```
This was called from 6f2fb6e0e7/src/clean.c (L1710).
(It is technically undefined behaviour to call strncmp with `NULL` pointers however).
