crypto: sha-mb - remove a bogus NULL check

This can't be NULL and we dereferenced it earlier.  Smatch used to
ignore these things where the pointer was obviously non-NULL but I've
found that sometimes the intention was to check something else so we
were maybe missing bugs.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>