git-add--interactive: Improve behavior on bogus input

git-add--interactive: Improve behavior on bogus input

1) Previously, any menu would cause a perl error when entered '0',
   which is never a valid option.

2) Entering a bogus choice (like 998 or 4-2) surprisingly caused
   the same behavior as if the user had just hit 'enter', which
   means to carry out the selected action on the selected items.
   Entering such bogus input is now a no-op and the sub-menu
   doesn't exit.

Signed-off-by: Jean-Luc Herren <jlh@gmx.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>