gpg: Avoid error exit if keygrip computations fails in a key listing.

* g10/keyid.c (keygrip_from_pk): Use log_info and clear array on error.

This may happen due to algorithms or curves not supported by
Libgcrypt. We should only print a warning and not have gpg return
with failure for a secret key listing.

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