When the user clicks on an equipped item, that item is 'temporarily'
placed on the floor, and so placed in a free slot of the item->item_list
dynarray.

The item is also 'hold in hand', via a pointer to the item_list's slot.
This pointer is called 'item_held_in_hand'.

When the hold-in-hand item is eventually dropped on the floor, a
new free storage slot is created in the level->item_list, the
temporary slot's content is moved into that new slot, and the
temporary slot, pointed to by item_held_in_hand, is cleared.

But, when the new free slot is created, the itemr_list dynarray may need
to be expanded, through a call to realloc(). The item_held_in_hand
pointer is then possibly no more pointing into the reallocated
dynarray, or even at a wrong place.

The 'temporary' item is now in an other memory location, and clearing
item_held_in_hand does no more clear the 'temporary' item.

This leads to a duplication of the dropped item, with the same content.

This patch ensures that the 'temporary' item is cleared before the
possible expansion of the dynarray.

Fixes #1002