fix OID DER_encoding in EC groups (HEAD/1_11)

Already signalled but still without a committed fix…

This code raises an exception:

#include <botan/ec_group.h>
#include <botan/oids.h>

int main() {
	const std::string name("secp256r1");
	const Botan::OID oid(Botan::OIDS::lookup(name));
	const Botan::EC_Group ecg(oid);

	std::vector<Botan::byte> der =
		ecg.DER_encode(Botan::EC_DOMPAR_ENC_OID);
}