Avoid using cnid = 0, use NID_undef instead, and return early instead of
trying to find an instance of that in the subject DN.
(Imported from upstrea's 40d5689458593aeca0d1a7f3591f7ccb48e459ac.)
Change-Id: I1bdf6bf7a4b1f4774a8dbec7e5df421b3a27c7e4
Reviewed-on: https://boringssl-review.googlesource.com/5947
Reviewed-by: Adam Langley <agl@google.com>