RT3065: ec_private_key_dont_crash

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-----

Reviewed-by: Dr Stephen Henson <steve@openssl.org>