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.

doc/fixes36.2

@@ -303,6 +303,7 @@ training riding skill had an off-by-one bug when counting turns riding
 message sequencing when vampire shifts to fog cloud to pass under closed door:
 	when in sight, give form change message before moving to door spot
 limit carrying heavy loads from water to land
+failing to carry a wished-for item behaved differenctly from dropping one
 
 
 Fixes to Post-3.6.1 Problems that Were Exposed Via git Repository