Some EC ASN.1 structures are using a named curve, but include the full
parameters anyway. With this change, BoringSSL will recognise the order
of the curve.

Change-Id: Iff057178453f9fdc98c8c03bcabbccef89709887
Reviewed-on: https://boringssl-review.googlesource.com/1270
Reviewed-by: Adam Langley <agl@google.com>

PR: 3338

(Imported from upstream's e34af3ec2b)

This change saves several EC routines from crashing when an EC_KEY is
missing a public key. The public key is optional in the EC private key
format and, without this patch, running the following through `openssl
ec` causes a crash:

-----BEGIN EC PRIVATE KEY-----
MBkCAQEECAECAwQFBgcIoAoGCCqGSM49AwEH
-----END EC PRIVATE KEY-----

Initial fork from f2d678e6e8 (1.0.2 beta).

(This change contains substantial changes from the original and
effectively starts a new history.)