Correct and improve part of previous todo-mode.el fix

Correct and improve part of previous todo-mode.el fix

* lisp/calendar/todo-mode.el (todo-jump-to-category): Improve code
by using bound-and-true-p.  This leaves a byte-compiler warning
unsilenced, but ideally, there shouldn't be a warning here (see
https://lists.gnu.org/archive/html/emacs-devel/2018-08/msg00131.html).
(todo--fifiles-history): New variable.
(todo-find-filtered-items-file): Use it to fix the filtered items
files history list for completing-read.