object deletion bookkeeping

object deletion bookkeeping

     After the apply-wielded-pie-to-blind-self (quite a while back) and
quivered-candle-burns-out (a week or two ago) bugs where object deletion
left dangling worn-object pointers, add a general check to try to catch
such things at the time of deletion.  Also, clear thrownobj and kickobj
pointers at the time corresponding object is deleted.  (This doesn't add
any checks for them becoming null during process of throwing or kicking,
where I'm not sure that the possibility of missile going away is being
adequately tracked).