The nticks calculation is still broken. Sometimes the delta systimer

The nticks calculation is still broken.  Sometimes the delta systimer
value is less then the periodic value.  This has been tracked down to
a 32 bit overflow, but 32 bit overflows are SOP for systimers.  The
actual bug is related to lwp_slptime.  lwp_slptime is supposed to detect
that an overflow might have occured but apparently it can wind up 0 in
certain cases which have yet to be tracked down.

For now just sanitize nticks.

Reported-by: "Simon 'corecode' Schubert" <corecode@fs.ei.tum.de>