length doesn't grow too large.
(sexp_iterator_next): Skip the current list only if type is
SEXP_LIST. Handle ')'.
(sexp_iterator_enter_list): Set type to SEXP_START.
(sexp_iterator_exit_list): Likewise. Don't skip the ')' here.
(sexp_iterator_assoc): Bug fix.

Rev: src/nettle/sexp.c:1.2