security: fix mbedtls_x509_crt_parse error

security: fix mbedtls_x509_crt_parse error

Add null terminator at the end of the cert, required by
mbedtls_x509_crt_parse.

This patch allows CT1.7.8.5 to make progress with its DTLS handshake.
Handshake fails later on, for different reasons, being investigated.

Change-Id: Iebfcf17be2661f080499961fbd259cef3b1c06d8
Signed-off-by: Dan Mihai <Daniel.Mihai@microsoft.com>
https://jira.iotivity.org/browse/IOT-2622
Reviewed-on: https://gerrit.iotivity.org/gerrit/21985
Reviewed-by: Kevin Kane <kkane@microsoft.com>
Reviewed-by: Way Vadhanasin <wayvad@microsoft.com>
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Nathan Heldt-Sheller <nathan.heldt-sheller@intel.com>