fix check for already-registered sessions in SyncUser::register_session
fix check for already-registered sessions in SyncUser::register_session

due to `m_sessions`/`m_waiting_sessions` being a map to `weak_ptr`,
there could be a key entry with no corresponding `SyncSession`.

realm/realm-core,realm/realm-core,realm/realm-object-store,realm/realm-core,realm/realm-object-store,realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-object-store,realm/realm-core