fixed dangling pointer using std::string

[randaz81]

Fix for: https://github.com/robotology/yarp/issues/3105 I'm aware that std::string is not a plain char*. ..but do we need to stick to this oooooold '99 style in libyarpmanager? I think not.
BTW, strings are used elsewhere in the same lib.

[update-docs[bot]]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would update the release notes by adding a file in doc/release/<target_branch>, based on your changes.

[sonarcloud[bot]]

Quality Gate failed

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)
29.0% Duplication on New Code (required ≤ 3%)

See analysis details on SonarCloud