Avoid minor waste in |ec_GFp_nistp256_point_get_affine_coordinates|.

Avoid calculating the affine Y coordinate when the caller didn't ask
for it, as occurs, for example, in ECDH.

For symmetry and clarity, avoid calculating the affine X coordinate in
the hypothetical case where the caller only asked for the Y coordinate.

Change-Id: I69f5993fa0dfac8b010c38e695b136cefc277fed
Reviewed-on: https://boringssl-review.googlesource.com/7590
Reviewed-by: David Benjamin <davidben@google.com>

crypto/ec/p256-64.c

@@ -1561,22 +1561,29 @@ static int ec_GFp_nistp256_point_get_affine_coordinates(const EC_GROUP *group,
   felem_inv(z2, z1);
   felem_square(tmp, z2);
   felem_reduce(z1, tmp);
-  felem_mul(tmp, x_in, z1);
-  felem_reduce(x_in, tmp);
-  felem_contract(x_out, x_in);
-  if (x != NULL && !smallfelem_to_BN(x, x_out)) {
-    OPENSSL_PUT_ERROR(EC, ERR_R_BN_LIB);
-    return 0;
+
+  if (x != NULL) {
+    felem_mul(tmp, x_in, z1);
+    felem_reduce(x_in, tmp);
+    felem_contract(x_out, x_in);
+    if (!smallfelem_to_BN(x, x_out)) {
+      OPENSSL_PUT_ERROR(EC, ERR_R_BN_LIB);
+      return 0;
+    }
   }
-  felem_mul(tmp, z1, z2);
-  felem_reduce(z1, tmp);
-  felem_mul(tmp, y_in, z1);
-  felem_reduce(y_in, tmp);
-  felem_contract(y_out, y_in);
-  if (y != NULL && !smallfelem_to_BN(y, y_out)) {
-    OPENSSL_PUT_ERROR(EC, ERR_R_BN_LIB);
-    return 0;
+
+  if (y != NULL) {
+    felem_mul(tmp, z1, z2);
+    felem_reduce(z1, tmp);
+    felem_mul(tmp, y_in, z1);
+    felem_reduce(y_in, tmp);
+    felem_contract(y_out, y_in);
+    if (!smallfelem_to_BN(y, y_out)) {
+      OPENSSL_PUT_ERROR(EC, ERR_R_BN_LIB);
+      return 0;
+    }
   }
+
   return 1;
 }