vtimer: fix crash on spurious hwtimer callback

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.