boulder/statue/landmine reversal and new fix

boulder/statue/landmine reversal and new fix
This includes a reversal of my earlier
boulder/statue/landmine fix,
and places a check for obj->where==OBJ_FLOOR into
fracture_rock().  I think this is a better approach
because:
- if eliminates the pointless extract/place in
fracture_rock, followed by extract/place in
the caller in the two places causing the crash
- it covers any similar situations that we
might have missed or that someone might add
accidentally (you might not expect the location
of an object to change inside fracture_rock())
- it allows fracturing to take place on the
other object chains if we ever need it (statues
falling down stairs, perhaps?)
- it doesn't move objects from other chains onto
the floor briefly as the current code does