gpg: Do not bail on an invalid packet in the local keyring.

* g10/keydb.c (parse_keyblock_image): Treat invalid packet special.

This is in particular useful to run --list-keys on a keyring with
corrupted packets. The extra flush is to keep the diagnostic close to
the regular --list-key output.

This is a backport from master with support for the unsupported v5 key
handling.

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