gpg: Fix segv due to NULL value stored as opaque MPI (BRANCH 2.0)

* g10/build-packet.c (do_secret_key): Check for NULL return from
gcry_mpi_get_opaque.
* g10/keyid.c (hash_public_key): Ditto.

This is a backport of 76c8122adfed0f0f443cce7bda702ba2b39661b3 from
master to the STABLE-BRANCH-2-0

On the STABLE-BRANCH-2-0, we may also want to patch g10/seckey-cert.c,
but that has not been done in this patch.

This fix extends commmit 0835d2f44ef62eab51fce6a927908f544e01cf8f.

gpg2 --export --no-default-keyring --keyring TESTDATA

With TESTDATA being below after unpacking.

-----BEGIN PGP ARMORED FILE-----

mBMEhdkMmS8BcX8F8F5voEhQAQmBMEnAAAZwAAo4D/f/8EhQAAAIAEnP8EhQAQ
iBMEnP8AAAAABf8jIID/8EhQYQmBMEnIUAEIgTBKT/AAAAAAUAACCA/f//BIUA
EJgTBJx/AP8ABPPzBJx/AP8ABPPz

2yE0

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

• Reported-by: Jodie Cunningham
• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>