Fix potential integer overflow parsing DER CRT

This patch prevents a potential signed integer overflow during the
certificate version verification checks.
diff --git a/ChangeLog b/ChangeLog
index 58ee285..567e988 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -228,6 +228,9 @@
      digits. Found and fixed by Guido Vranken.
    * Fix unlisted DES configuration dependency in some pkparse test cases. Found
      by inestlerode. #555
+   * Fix a potential integer overflow in the version verification for DER
+     encoded X509 certificates. The overflow would enable maliciously
+     constructed certificates to bypass the certificate verification check.
 
 = mbed TLS 2.4.1 branch released 2016-12-13