master 50765f3f511d46: Make run-at-time try harder to run at integral multiples

[Ergus]

Hi:

On current master I am getting an issue after 50765f3f511d46.

Emacs hangs after start and I get this message after some seconds:

Error running timer ‘jit-lock-stealth-fontify’: (wrong-type-argument
timerp [t nil nil nil nil nil nil nil nil]) [16 times]

When enable debug-on-error on the beginning I get this backtrace:

Debugger entered--Lisp error: (wrong-type-argument timerp [t nil nil nil nil 
nil nil nil nil])
  timer-set-function([t nil nil nil nil nil nil nil nil] 
jit-lock-stealth-fontify (t))
  jit-lock-mode(t)
  jit-lock-register(font-lock-fontify-region t)
  font-lock-turn-on-thing-lock()
  font-lock-mode-internal(t)
  font-lock-default-function(t)
  font-lock-mode()
  turn-on-font-lock()
  turn-on-font-lock-if-desired()
  global-font-lock-mode-enable-in-buffers()
  run-hooks(after-change-major-mode-hook)
  run-mode-hooks(lisp-interaction-mode-hook)
  lisp-interaction-mode()
  command-line()
  normal-top-level()

emacs -Q does not have this issue.

But in an emacs config without external packages
[https://github.com/Ergus/mini_dotemacs] I get the same error; so the
issue is not related with broken external dependencies.

If I have some time I'll try to get more information, but maybe this
info is enough for you?

Best,
Ergus