Use _bytes/nul-terminated, not _pointer w.r.t. appropriate char* structure fields

[zyrolasting]

Credit to @dalev for observation in #27

Fwiw, I took a quick look at the copy of vk.xml in this repo, and there are both param and member elements with attribute len="null-terminated". I suspect that if that attribute is present, and the type is char*, then _bytes/nul-terminated would be an appropriate FFI type for the parameter or field in question. But I haven't tried it; it may not be so easy.