(#21869) Fix recursion in cert expiration check

During every authenticated request, the expiration date of the involved
certificates is checked. However, if the localhost cert is loaded when the CA
cert is not present an authenticated request will be initiated to download the
CA cert. This triggers another expiration check for authenticated requests,
which loads the localhost cert, which initiates another authenticated request
to download the CA cert... and so on until stack space is exhausted.

This patch skips the expiration check for the localhost cert if the CA cert is
missing.

[Redmine issue #21869](http://projects.puppetlabs.com/issues/21869)