crypto: af_alg - rewrite NULL pointer check

Because of the possible alloc failure of the alloc_page(), it could
return NULL pointer.
And there is a check below the sg_assign_page().
But it will be more logical to move the NULL check before the
sg_assign_page().

Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>