gpg: Fix a NULL-deref in export due to invalid packet lengths.

* g10/build-packet.c (write_fake_data): Take care of a NULL stored as
opaque MPI.

Test data:

gpg2 --no-default-keyring --keyring FILE --export

With this unpacked data for FILE:

-----BEGIN PGP ARMORED FILE-----
Version: GnuPG v2
Comment: Use "gpg --dearmor" for unpacking

mI0EGRkZGRkZGRkZGRkZGRkBGRkZGRkZGRkZGRkZGQAZGRkZGRkZGRkZGRkZGRkZ
GRkZInzgDbpa/9gQ4wq9////f3Vy81CkyVq3HQaqgZLZOeqPjM7XgGh5hJvAkpec
9wAAAgDHe0FVFbkppJZXP+gFs6z3gobS0qUFeyTtxs+wAgAD

JDFT

-----END PGP ARMORED FILE-----

• Reported-by: Hanno Böck <hanno@hboeck.de>
• Signed-off-by: Werner Koch <wk@gnupg.org>