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>

crypto/af_alg.c

@@ -931,16 +931,19 @@ int af_alg_sendmsg(struct socket *sock, struct msghdr *msg, size_t size,
 			sg_unmark_end(sg + sgl->cur - 1);
 
 		do {
+			struct page *pg;
 			unsigned int i = sgl->cur;
 
 			plen = min_t(size_t, len, PAGE_SIZE);
 
-			sg_assign_page(sg + i, alloc_page(GFP_KERNEL));
-			if (!sg_page(sg + i)) {
+			pg = alloc_page(GFP_KERNEL);
+			if (!pg) {
 				err = -ENOMEM;
 				goto unlock;
 			}
 
+			sg_assign_page(sg + i, pg);
+
 			err = memcpy_from_msg(page_address(sg_page(sg + i)),
 					      msg, plen);
 			if (err) {