Handle cases when gipfl/linux-health returns false instead of memory …

[trukonsult]

The module gipfl/linux-health, which Directory uses to read memory usage for the background daemon, returns false if the system doesn't have a /proc-filesystem. This causes the WebUI to fail with an error. This patch allows the WebUI to work and instead shows "n/a" for memory usage in such case.

[cla-bot]

Thank you for your pull request. Before we can look at it, you'll need to sign a Contributor License Agreement (CLA).

Please follow instructions at https://icinga.com/company/contributor-agreement to sign the CLA.

After that, please reply here with a comment and we'll verify.

Contributors that have not signed yet: @trukonsult

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Please contact us if you think this is the case.

• If you signed the CLA as a corporation, your GitHub username may not have been submitted to us. Please reach out to the responsible person in your organization.

[bobapple]

@cla-bot check

[Thomas-Gelf]

Merged, thank you!