Crash when documenting C files

Hi,

I enabled C files in ford 6.0.0 by specifying

extra_filetypes: c //

in the project filed. Unfortunately ford crashes with an AttributeError when processing the first C file. I don't know if there is something wrong with that file or if this is an internal problem with ford.

For reference I append in [files.zip](https://github.com/Fortran-FOSS-Programmers/ford/files/2314459/files.zip)

- the project file, hispeet.md
- the C file, parmetis_wrapper.c (just 32 lines)
- the output of ford including the error message, ford.stdout
- the used css style file, hispeet.css (for completeness)

Thanks in advance for any helpful comments.

Jörg