CVE-2018-16841 heimdal: Fix segfault on PKINIT with mis-matching prin…

…cipal

In Heimdal KRB5_KDC_ERR_CLIENT_NAME_MISMATCH is an enum, so we tried to double-free
mem_ctx.

This was introduced in 9a0263a for the
MIT KDC effort.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13628

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>

source4/kdc/db-glue.c

@@ -2610,10 +2610,10 @@ samba_kdc_check_pkinit_ms_upn_match(krb5_context context,
 	 * comparison */
 	if (!(orig_sid && target_sid && dom_sid_equal(orig_sid, target_sid))) {
 		talloc_free(mem_ctx);
-#ifdef KRB5_KDC_ERR_CLIENT_NAME_MISMATCH /* Heimdal */
-		return KRB5_KDC_ERR_CLIENT_NAME_MISMATCH;
-#elif defined(KRB5KDC_ERR_CLIENT_NAME_MISMATCH) /* MIT */
+#if defined(KRB5KDC_ERR_CLIENT_NAME_MISMATCH) /* MIT */
 		return KRB5KDC_ERR_CLIENT_NAME_MISMATCH;
+#else /* Heimdal (where this is an enum) */
+		return KRB5_KDC_ERR_CLIENT_NAME_MISMATCH;
 #endif
 	}