When --throw-keyids, --hidden-recipient, or --hidden-encrypt-to were
used for an encrypted file, the encryption metadata uses all zeroes as
the key ID. Ignore these key IDs to avoid generating extraneous
messages during decryption or adding invalid IDs to the recipients list.
Closesjamessan/vim-gnupg#50
Signed-off-by: James McCoy <vega.james@gmail.com>