Fix crash in auth tests by removing implicit User constructor from AuthData

pointer.

PiperOrigin-RevId: 246573138

Author: jonsimantov

auth/src/data.h

@@ -90,7 +90,6 @@ struct AuthData {
 
     app = nullptr;
     auth = nullptr;
-    current_user = nullptr;
     auth_impl = nullptr;
     user_impl = nullptr;
     listener_impl = nullptr;

auth/src/include/firebase/auth/user.h

@@ -439,7 +439,7 @@ class User : public UserInfoInterface {
  private:
   friend struct AuthData;
   // Only exists in AuthData. Access via @ref Auth::CurrentUser().
-  User(AuthData* auth_data) : auth_data_(auth_data) {}
+  explicit User(AuthData* auth_data) : auth_data_(auth_data) {}
 
 #if defined(INTERNAL_EXPERIMENTAL)
   // Doxygen should not make docs for this function.