[PATCH 1/3] lib: fix NULL checks for filenames iterators

[David Bremner]

Sascha Silbe <sascha-pgp at silbe.org> writes:

> The API documentation (notmuch.h) states that the parameter may be NULL,
> but the implementation only checked the current element, potentially
> dereferencing a NULL pointer in the process.
>

Pushed this one patch.

d