Bug 5236 Followup: items table information being keyed by biblionumber instead!

Bug 5236 Followup: items table information being keyed by biblionumber instead!

This patch fixes an error with bugfix 5236; any item-table information in the PREDUE letter
was being parsed with biblionumber as the key, instead of itemnumber.  Unless itemnumber == biblionumber,
this will ALWAYS return the wrong information.

I've moved the item table parsing line to within the if ($itemnumber) conditional check, and replaced
the key to use the itemnumber instead of the biblionumber.

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>