[CRYPTO] scatterwalk: Handle zero nbytes in scatterwalk_map_and_copy

It's better to return silently than crash and burn when someone feeds us
a zero length.  In particular the null digest algorithm when used as part
of authenc will do that to us.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --cc crypto/scatterwalk.c
Simple merge