[menu] Prevent character code zero from acting as a shortcut key

Unrecognised keys may be returned by getkey() as character code zero,
which currently matches against the first menu item with no shortcut
key defined.

Prevent this unintended behaviour by explicitly checking that the menu
item has a defined shortcut key.

Reported-by: Ján ONDREJ (SAL) <ondrejj@salstar.sk>
Signed-off-by: Michael Brown <mcb30@ipxe.org>