Fix build error.

Author: jonsimantov

firestore/testapp/src/common_main.cc

@@ -127,7 +127,7 @@ extern "C" int common_main(int argc, const char* argv[]) {
   Await(login_future, "Auth sign-in");
   auto* login_result = login_future.result();
   if (login_result) {
-    const firebase::auth::AuthResult user = *login_result.user;
+    const firebase::auth::User user = login_result->user;
     LogMessage("Signed in as %s user, uid: %s, email: %s.\n",
                user->is_anonymous() ? "an anonymous" : "a non-anonymous",
                user->uid().c_str(), user->email().c_str());