gpg: Avoid a double free on error in the key generation.

* g10/keygen.c (card_store_key_with_backup): Avoid double free and
simplify error handling.

This is part of

(cherry picked from commit bcc002cd45d1c6bd51c2b2093f92d396970c082e)

• GnuPG-bug-id: T7129
• Co-authored-by: Jakub Jelen <jjelen@redhat.com>