ec/ecp_nistz256.c: fix ecp_nistz256_set_from_affine.

ecp_nistz256_set_from_affine is called when application attempts to use
custom generator, i.e. rarely. Even though it was wrong, it didn't
affect point operations, they were just not as fast as expected.
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6738)

crypto/ec/ecp_nistz256.c

@@ -1104,28 +1104,12 @@ __owur static int ecp_nistz256_set_from_affine(EC_POINT *out, const EC_GROUP *gr
                                                const P256_POINT_AFFINE *in,
                                                BN_CTX *ctx)
 {
-    BIGNUM *x, *y;
-    BN_ULONG d_x[P256_LIMBS], d_y[P256_LIMBS];
     int ret = 0;
 
-    x = BN_new();
-    if (x == NULL)
-        return 0;
-    y = BN_new();
-    if (y == NULL) {
-        BN_free(x);
-        return 0;
-    }
-    memcpy(d_x, in->X, sizeof(d_x));
-    bn_set_static_words(x, d_x, P256_LIMBS);
-
-    memcpy(d_y, in->Y, sizeof(d_y));
-    bn_set_static_words(y, d_y, P256_LIMBS);
-
-    ret = EC_POINT_set_affine_coordinates_GFp(group, out, x, y, ctx);
-
-    BN_free(x);
-    BN_free(y);
+    if ((ret = bn_set_words(out->X, in->X, P256_LIMBS))
+        && (ret = bn_set_words(out->Y, in->Y, P256_LIMBS))
+        && (ret = bn_set_words(out->Z, ONE, P256_LIMBS)))
+        out->Z_is_one = 1;
 
     return ret;
 }
@@ -1181,9 +1165,9 @@ __owur static int ecp_nistz256_points_mul(const EC_GROUP *group,
             if (pre_comp_generator == NULL)
                 goto err;
 
+            ecp_nistz256_gather_w7(&p.a, pre_comp->precomp[0], 1);
             if (!ecp_nistz256_set_from_affine(pre_comp_generator,
-                                              group, pre_comp->precomp[0],
-                                              ctx)) {
+                                              group, &p.a, ctx)) {
                 EC_POINT_free(pre_comp_generator);
                 goto err;
             }