When an aead constructed through crypto_nivaead_default fails
its selftest, we'll loop forever trying to construct new aead
objects but failing because it already exists.

The crux of the issue is that once an aead fails the selftest,
we'll ignore it on the next run through crypto_aead_lookup and
attempt to construct a new aead.

We should instead return an error to the caller if we find an
an that has failed the test.

This bug hasn't manifested itself yet because we don't have any
test vectors for the existing nivaead algorithms.  They're tested
through the underlying algorithms only.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>