crypto: talitos - avoid out of bound scatterlist iterator

Check return value of scatterlist_sg_next(), i.e. don't rely solely
on number of bytes to be processed or number of scatterlist entries.

Signed-off-by: Horia Geanta <horia.geanta@freescale.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>