Fix recently introduced bool vector overrun.

Fix recently introduced bool vector overrun.

This was due to an optimization that went awry.
Reported by Glenn Morris in
<http://lists.gnu.org/archive/html/emacs-devel/2013-11/msg00622.html>.
* alloc.c (make_uninit_bool_vector): Don't allocate a dummy word
for empty vectors, undoing the 2013-11-18 change.
* data.c (bool_vector_binop_driver): Rely on this.
Fix bug that occasionally overran the destination.
* lisp.h (struct Lisp_Bool_vector): Document this.