Security
   * Zeroising of local buffers and variables which are used for calculations
     in mbedtls_pkcs5_pbkdf2_hmac(), mbedtls_internal_sha*_process(),
     mbedtls_internal_md*_process() and mbedtls_internal_ripemd160_process()
     functions to erase sensitive data from memory. Reported by
     Johan Malmgren and Johan Uppman Bruce from Sectra.
