crypto: authenc - Fix zero-length IV crash

commit 29b37f42127f7da511560a40ea74f5047da40c13 upstream.

As it is if an algorithm with a zero-length IV is used (e.g.,
NULL encryption) with authenc, authenc may generate an SG entry
of length zero, which will trigger a BUG check in the hash layer.

This patch fixes it by skipping the IV SG generation if the IV
size is zero.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>