fix #H6942 - dropx vs hold_another_object

fix #H6942 - dropx vs hold_another_object

Dropping an existing fragile item while levitating will usually
break it.  Getting a new wished-for fragile item and dropping it
because of fumbling or overfull inventory never would.

Some callers of hold_another_object() held on to its return value,
others discarded that.  That return value was unsafe if the item
was dropped and fell down a hole (or broke [after this change]).
Return Null if we can't be sure of the value, and make sure all
callers are prepared to deal with Null.