DWL YAML wrapper not working with Ubuntu's system v 5.2

[mcamurri]

Apparently, even the official libyaml-cpp is broken in Ubuntu 16.04

When compiling DWL and running the yaml_sample, I get a std::bad_alloc error.

This happens at this assignment and I have no better luck when running the rest of the code.

Users report similar issues here:
https://bugs.launchpad.net/ubuntu/+source/yaml-cpp/+bug/1646914

When installing the newest stable version 0.6.2 (which is also boost independent) I have no issues with DWL, but there are issues with other components which use DWL.

Ubuntu 16.04 is officially the worst LTS ever 👎

Anyhow, there is not much sanitization on that wrapper function: no check whether the key exists or if the node is a map. And it also overwrite the argument with a part of itself, which sounds dangerous.

Better to take a const reference of the argument and return a new child node instead.