crypto: omap-sham - huge buffer access fixes

The ctx internal buffer can only hold buflen amount of data, don't try
to copy over more than that. Also, initialize the context sg pointer
if we only have data in the context internal buffer, this can happen
when closing a hash with certain data amounts.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>