RC4: fixed ContinuousBuffer() using MODE_MCRYPT

Hi Terra, as mentioned... here's the bugfix for RC4 using multiple calls to ContinuousBuffer() in MODE_MCRYPT

Example:
```php
define('CRYPT_RC4_MODE',CRYPT_RC4_MODE_MCRYPT); // CRYPT_RC4_MODE_INTERNAL works fine

$cipher = new Crypt_RC4();
$cipher->enableContinuousBuffer();
$cipher->encrypt('1');
$cipher->disableContinuousBuffer();

$cipher->enableContinuousBuffer();
echo bin2hex($cipher->encrypt('1'))."\n";

$cipher = new Crypt_RC4();
echo bin2hex($cipher->encrypt('1'));
```

Output:
29
ef

Expected:
ef
ef

Output after bugfix:
ef
ef