Correct c-struct 'null' checks, fix failing tests due to expired certs, improve openssl error handling

[euank]

This PR does a lot because, well, to make the tests not fail due to frankdd certs that expired in late 2017, a lot of stuff was required.

The main point of this PR, however, is the "Correct c-struct 'null' checks" commit. The use of if not c_struct to check whether something was null didn't work, and so it was fairly easy to end up with memory unsafe behaviour when anything other than the completely happy path was hit in the openssl integration.

cc @SkyLothar

[cdbattags]

Ope, just now seeing this! Adding this to my new fork at https://github.com/cdbattags/lua-resty-jwt.