sasl_sspi: Free the Kerberos V5 challenge as soon as we're done with it

Reduced the amount of free's required for the decoded challenge message
in Curl_sasl_create_gssapi_security_message() as a result of coding it
differently in the sasl_gssapi module.
sasl_sspi: Don't use hard coded sizes in Kerberos V5 security data

Don't use a hard coded size of 4 for the security layer and buffer size
in Curl_sasl_create_gssapi_security_message(), instead, use sizeof() as
we have done in the sasl_gssapi module.
sasl_sspi: Corrected some typos