Reset item_tester_hook in spell_identify_unknown_available() to avoid strange effects later
All other uses of item_tester_hook are followed either by get_item(), or by
scan_items(). Both of those functions reset the hook once they've used it.
Fixes #1818
Merge pull request #371 from pnd10/3.5-release

Reset item_tester_hook in spell_identify_unknown_available() to avoid strange effects later