Bugfix
   * In configurations with MBEDTLS_SSL_DTLS_CONNECTION_ID enabled but none of
     MBEDTLS_SSL_HW_RECORD_ACCEL, MBEDTLS_SSL_EXPORT_KEYS or MBEDTLS_DEBUG_C,
     DTLS handshakes using CID would crash due to a null pointer dereference.
     Fix this. Fixes #3998.