crypto: mv_cesa - make count_sgs() null-pointer proof

This also makes the dummy scatterlist in mv_hash_final() needless, so
drop it.

XXX: should this routine be made pulicly available? There are probably
other users with their own implementations.

Signed-off-by: Phil Sutter <phil.sutter@viprinet.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>