Fix EC_KEY_set_private_key() to call key->group->meth->set_private()

Fix a bug introduced by 6903e2e7e9a4 (Extended EC_METHOD customisation
support., 2016-02-01). key->meth->set_private() is wrongly called where
it should call key->group->meth->set_private().

PR#4517

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Stephen Henson <steve@openssl.org>
