gpg: Properly account for ring trust packets.

* g10/keyring.c (keyring_get_keyblock): Use the parser's packet count
instead of counting ourself.
* g10/packet.h (struct parse_packet_ctx_s): New field
'n_parsed_packets'.
(init_parse_packet): Initialize new field.
* g10/parse-packet.c (parse): Count packets.

The 'keyring' keystore depends on the number of packets for delete and
update operations. With the rework of the ring trust packets, the
trust packets were no longer properly accounted for leading to keyring
corruptions.

The 'keybox' store was not affected.

• GnuPG-bug-id: T3144
• Fixes-commit: a8895c99a7d0750132477d80cd66caaf3a709113
• Signed-off-by: Justus Winter <justus@g10code.com>