bstr is a bounded string type, consisting of a pointer and a length
value. If the length is 0, the pointer can be NULL. This is somewhat
logical due to how this abstraction works, but it can leak when
converting to C strings.
talloc_strndup() returns NULL instead of "" in this case, which broke
some other code. Use bstrto0() instead, which is the "proper" function
to convert bstr to char*.
Fixes#1462.
8144d142e4