Fix Octo-Tiger segfaults when using HPX master

[G-071]

Using HPX master uncovered an issue within Octo-Tiger causing segfaults during the initialization (as outlined in the original HPX issue STEllAR-GROUP/hpx#6414 and the PR STEllAR-GROUP/hpx#6415)

This should hotfix the issue for now (at least the local tests pass again)!
However, in the future we will want to remove this part of the code and use shared_ptr instead via

future<std::shared_ptr<node_server>> node_client::get_ptr() const {
    return hpx::get_ptr<node_server>(get_unmanaged_gid());
}

to properly fix this.

[G-071]

Closed due to #479 providing a better fix!