Always allocate at least one bits_word per bool vector.

Always allocate at least one bits_word per bool vector.

See Daniel Colascione in:
http://lists.gnu.org/archive/html/emacs-devel/2013-11/msg00518.html
* alloc.c (make_uninit_bool_vector): Always allocate at least one word.
* data.c (bool_vector_binop_driver): Rely on this.  Tune.
* lisp.h (struct Lisp_Bool_vector): Document this.