Prevent signed integer overflow in CSR parsing

Modify the function mbedtls_x509_csr_parse_der() so that it checks the
parsed CSR version integer before it increments the value. This prevents
a potential signed integer overflow, as these have undefined behaviour
in the C standard.

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index 567e988..eea6919 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -46,6 +46,10 @@
      Reported and fix suggested by guidovranken in #740
    * Fix conditional preprocessor directives in bignum.h to enable 64-bit
      compilation when using ARM Compiler 6.
+   * Fix potential integer overflow in the version verification for DER
+     encoded X509 CSRs. The overflow would enable maliciously constructed CSRs
+     to bypass the version verification check. Found by Peng Li/Yueh-Hsun Lin,
+     KNOX Security, Samsung Research America
 
 Security
    * Fix authentication bypass in SSL/TLS: when auth_mode is set to optional,