func bmpString(s string) ([]byte, error) {
	ret := make([]byte, 0, 2*len(s)+2)
	for _, r := range s {
		if t, _ := utf16.EncodeRune(r); t != 0xfffd {
			return nil, errors.New("go-pkcs12: string contains characters that cannot be encoded in UCS-2")
		}
		ret = append(ret, byte(r/256), byte(r%256))
	}
	return append(ret, 0, 0), nil
}
bmpString returns s encoded in UCS-2 with a zero terminator.
package pkcs12
import (
	"errors"
	"unicode/utf16"
)