scd: Slight change to app->fnc.do_with_keygrip.

* scd/app-openpgp.c (do_with_keygrip): Return a real error code to
avoid misinterpretation of the result.  Also fix the case for a too
small buffer.

The only real chnage is the case for a too small buffer. That should
in general never happen but if so we now return an error instead of
success.

• Signed-off-by: Werner Koch <wk@gnupg.org>