RT3065: automatically generate a missing EC public key

When d2i_ECPrivateKey reads a private key with a missing (optional) public key,
generate one automatically from the group and private key.

Reviewed-by: Dr Stephen Henson <steve@openssl.org>
(cherry picked from commit ed383f847156940e93f256fed78599873a4a9b28)

Conflicts:
	doc/crypto/EC_KEY_new.pod