When the first call to af_alg_make_sg fails, we may return garbage
instead of the real error.  This patch fixes it by setting the error
if "copied" is zero.

Based on a patch by Jesper Juhl.
Reported-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>