sue will happen when entering menu "FOO" and requesting help.

nconf is the only front-end which do not filter the validity of the
current menu. Such filter can not really happen beforehand as other key
which does not deals with the current menu might be entered by the user,
so just bails out earlier if we encounter an invalid menu.

Cc: Nir Tzachar <nir.tzachar@gmail.com>
Cc: Andrej Gelenberg <andrej.gelenberg@udo.edu>
Reported-by: Andrej Gelenberg <andrej.gelenberg@udo.edu>
Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>
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