krb5: gss_process_context_token() incorrectly frees context (MITKRB5-SA-2015-001)

A use-after-free flaw was found in the way the MIT Kerberos libgssapikrb5 library processed valid context deletion tokens. An attacker able to make an application using the GSS-API library libgssapi call the gssprocesscontexttoken function could use this flaw to crash that application...