Fix null deref in SPNEGO acceptor [CVE-2014-4344]

When processing a continuation token, acc_ctx_cont was dereferencing
the initial byte of the token without checking the length.  This could
result in a null dereference.

CVE-2014-4344:

In MIT krb5 1.5 and newer, an unauthenticated or partially
authenticated remote attacker can cause a NULL dereference and
application crash during a SPNEGO negotiation by sending an empty
token as the second or later context token from initiator to acceptor.
The attacker must provide at least one valid context token in the
security context negotiation before sending the empty token.  This can
be done by an unauthenticated attacker by forcing SPNEGO to
renegotiate the underlying mechanism, or by using IAKERB to wrap an
unauthenticated AS-REQ as the first token.

    CVSSv2 Vector: AV:N/AC:L/Au:N/C:N/I:N/A:C/E:POC/RL:OF/RC:C

[kaduk@mit.edu: CVE summary, CVSSv2 vector]

(cherry picked from commit 524688ce87a15fc75f87efc8c039ba4c7d5c197b)

ticket: 7970
version_fixed: 1.12.2
status: resolved
spnego_gss_acquire_cred(OM_uint32 *minor_status,
			gss_name_t desired_name,
			OM_uint32 time_req,
			gss_OID_set desired_mechs,
			gss_cred_usage_t cred_usage,
			gss_cred_id_t *output_cred_handle,
			gss_OID_set *actual_mechs,
			OM_uint32 *time_rec)
{
    return spnego_gss_acquire_cred_from(minor_status, desired_name, time_req,
					desired_mechs, cred_usage, NULL,
					output_cred_handle, actual_mechs,
					time_rec);
}

spnego_gss_acquire_cred(OM_uint32 *minor_status,
			gss_name_t desired_name,
			OM_uint32 time_req,
			gss_OID_set desired_mechs,
			gss_cred_usage_t cred_usage,
			gss_cred_id_t *output_cred_handle,
			gss_OID_set *actual_mechs,
			OM_uint32 *time_rec)
{
    return spnego_gss_acquire_cred_from(minor_status, desired_name, time_req,
					desired_mechs, cred_usage, NULL,
					output_cred_handle, actual_mechs,
					time_rec);
}
