crypto: hash - initialize entry len for null input in crypto hash sg list walk

For the special case when we have a null input string, we want
to initialize the entry len to 0 for the hash/ahash walk, so
cyrpto_hash_walk_last will return the correct result indicating
that we have completed the scatter list walk.  Otherwise we may
keep walking the sg list and access bogus memory address.

Change-Id: I8dee60c77bbffb257164d41b40a031b283957ac7
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>