e1705622b7
It seems that there are corner cases where a vtimer was removed, but still there comes a hwtimer callback. This is a bug somewhere in the vtimer or hwtimer. If there still was a vtimer set, then the next one gets called before its time. If there was no other vtimer scheduled, then `timer->action(timer)` crashes. This PR simply fixes the crash, but does not attempt to find the more fundamental bug.