gpg: Fix possible double free of the card serialno.

* g10/free-packet.c (copy_public_key): Copy fields serialno and
updateurl.

The PK->serialno is used to get the version of the card to decide
whether it does support other algorithms than SHA-1. This value is
cached but no deep copy was done when calling copy_public_key.

Bug detected by importing some public keys and then importing a secret
key which led to a double free.

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