iuse::knife: menu() didn't end in NULL but only crashed when built with -O. Also, remembers last menu selection (runtime).

Fixes https://github.com/CleverRaven/Cataclysm-DDA/issues/2948
Addresses https://github.com/CleverRaven/Cataclysm-DDA/pull/2933#issuecomment-24113794 (hopefully)