kconfig: fix segv fault in menuconfig

With specific configurations requesting help for certain
menu lines caused menuconfig to crash.
This was tracked down to a null pointer bug.
Thanks to "Miles Lane" <miles.lane@gmail.com> for inital reporting
and to Gabriel C <nix.or.die@googlemail.com> for the backtrace
that helped me locating the bug.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>