Fix an incoherent test.

Pointer 'o' is set inside a local buffer, so it can't be NULL.
Also fix coding style and add comments

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4754)