* g10/packet.h (PKT_symkey_enc): Allocate the session key so that we
have a fixed length for this struct.
* g10/free-packet.c (free_symkey_enc): Adjust for this change
* g10/parse-packet.c (parse_symkeyenc): Ditto.
* g10/encrypt.c (encrypt_simple, write_symkey_enc): Ditto.