[devel at bb.net] Mergeable Buildbot pull requests

[dustin at buildbot.net]

Mergeable pull requests:

https://github.com/buildbot/buildbot/pull/2521 - fix CID 1382001: Attribute/item access or function call before check for None or undefined
https://github.com/buildbot/buildbot/pull/2514 - DockerLatentWorker: fix threading issues
https://github.com/buildbot/buildbot/pull/2507 - fixes GitHub status 403 error
https://github.com/buildbot/buildbot/pull/2505 - httpclientservice: do not use urljoin