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.

(Imported from upstream's 2083f7c465d07867dd9867b8742bb71c03d1f203)

Change-Id: I9e5090de87cf846ab92e4be5b6bf64e6091d02e4