Fix problem with re-adding timer in callback

[tidklaas]

When processing timer callbacks, atomTimerCallbacks() removes due
timers from the timer_queue and puts them in a callback list.
If a callback calls atomTimerRegister() for itself, the timer gets
prepended to the timer_queue. On return, the callback processing
will continue in the timer_queue instead of the callback list.
Fixed by saving a pointer to the next entry in the callback list
before calling the cb_func.

Fixes #17

[tidklaas]

Oops, @simonccn already submitted a fix for this.