gpg: Do not abort on certain invalid packets.

* g10/build-packet.c (write_fake_data): Check for non-opaque data.
* g10/seskey.c (do_encode_md): Return NULL instead of abort.

The first may happen if the usage flags of an algorithm do not match
the allowed usage. When writing a backsig this would lead to a
log_bug in libgcrypt due to the use of a regular MPI as opaque data.

The second may happen with all kind of invalid data. It is easy to
avoid an abort, though.

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