fix for handling KERB_ERROR's

[0xe7]

My last PR included handling of e_data within KERB_ERROR's for retrieval of the account salt from PA Data returned when a TGT was requested without preauth. This broke some errors (for instance AP_ERR_MODIFIED) where e_data isn't PA Data sections, as shown below:

[*] Action: Ask TGS

[*] Requesting default etypes (RC4_HMAC, AES[128/256]_CTS_HMAC_SHA1) for the service ticket
[*] Building TGS-REQ request for: 'ldap/idc1.internal.zeroday.lab'
[*] Using domain controller: idc1.internal.zeroday.lab (192.168.71.20)

[!] Unhandled Rubeus exception:

System.NullReferenceException: Object reference not set to an instance of an object.
   at Rubeus.PA_DATA..ctor(AsnElt body)
   at Rubeus.KRB_ERROR..ctor(AsnElt body)
   at Rubeus.Ask.TGS(String userName, String domain, Ticket providedTicket, Byte[] clientKey, KERB_ETYPE paEType, String service, KERB_ETYPE requestEType, String outfile, Boolean ptt, String domainController, Boolean display, Boolean enterprise, Boolean roast, Boolean opsec, KRB_CRED tgs, String targetDomain, String servicekey, String asrepkey, Boolean u2u, String targetUser, Boolean printargs, String proxyUrl)
   at Rubeus.Ask.TGS(KRB_CRED kirbi, String service, KERB_ETYPE requestEType, String outfile, Boolean ptt, String domainController, Boolean display, Boolean enterprise, Boolean roast, Boolean opsec, KRB_CRED tgs, String targetDomain, String servicekey, String asrepkey, Boolean u2u, String targetUser, Boolean printargs, String proxyUrl)
   at Rubeus.Commands.Asktgs.Execute(Dictionary`2 arguments)
   at Rubeus.Domain.CommandCollection.ExecuteCommand(String commandName, Dictionary`2 arguments)
   at Rubeus.Program.MainExecute(String commandName, Dictionary`2 parsedArgs)

Fixed by putting a try/catch around decoding the e_data section and setting it to null if it fails:

[*] Action: Ask TGS

[*] Requesting default etypes (RC4_HMAC, AES[128/256]_CTS_HMAC_SHA1) for the service ticket
[*] Building TGS-REQ request for: 'ldap/idc1.internal.zeroday.lab'
[*] Using domain controller: idc1.internal.zeroday.lab (192.168.71.20)

[X] KRB-ERROR (41) : KRB_AP_ERR_MODIFIED