Raise an error in FindKey when returning a nil empty key

[nicop311]

When FindKey methods return an empty (nil) key, they should return an error Error: could not find a key with ID and Label ..., and not return nil, nil.

This should be modified in the following methods:

• crypto11/keys.go

  Lines 246 to 261 in 9fba4fe

  	func (c *Context) FindKeyPair(id []byte, label []byte) (Signer, error) {
  	if c.closed.Get() {
  	return nil, errClosed
  	}
  	result, err := c.FindKeyPairs(id, label)
  	if err != nil {
  	return nil, err
  	}
  	if len(result) == 0 {
  	return nil, nil
  	}
  	return result[0], nil
  	}

• crypto11/keys.go

  Line 303 in 9fba4fe

  func (c *Context) FindKeyPairWithAttributes(attributes AttributeSet) (Signer, error) {

• crypto11/keys.go

  Line 398 in 9fba4fe

  func (c *Context) FindKey(id []byte, label []byte) (*SecretKey, error) {

• crypto11/keys.go

  Line 446 in 9fba4fe

  func (c *Context) FindKeyWithAttributes(attributes AttributeSet) (*SecretKey, error) {

• crypto11/keys.go

  Line 561 in 9fba4fe

  func (c *Context) FindPrivateKey(id []byte, label []byte) (PrivateKey, error) {

• crypto11/keys.go

  Line 610 in 9fba4fe

  func (c *Context) FindPrivateKeyWithAttributes(attributes AttributeSet) (PrivateKey, error) {

• crypto11/rsa.go

  Line 213 in 9fba4fe

  func (c *Context) FindRSAPrivateKey(id []byte, label []byte) (RSAPrivateKey, error) {

• crypto11/rsa.go

  Line 345 in 9fba4fe

  func (c *Context) FindRSAKeyPair(id []byte, label []byte) (SignerDecrypter, error) {