stale vptrs for obj->{nexthere,ocontainer,ocarry}

stale vptrs for obj->{nexthere,ocontainer,ocarry}

'struct obj' contains a union of mutually exclusive pointers, but
removing an obj from a list wasn't clearing whichever one had been
in use.  If something is removed from a monster's inventory, clear
the object's pointer back to that monster; if something is removed
from a container, clear the object's pointer back to that container;
and whenever something is removed from the floor, clear the pointer
to the object which followed it at that floor location.