Fix a read off the end of a node BIO buffer that was calling a failure to read and parse X509 certificates...

Which was in turn causing the 'ca' option to `https.Agent` to be ignored.

This bug was introduced in a58f93ffc4a23aa7240808288acf8cf9f3022abea

A gist containing git bisection logs and a test case can be found here: https://gist.github.com/maxtaco/9211605