ssl_tls12_server: fix potential NULL-dereferencing if local certificate was not set.
ssl_tls12_server: fix potential NULL-dereferencing if local certificate was not set.

Signed-off-by: Leonid Rozenboim <0712c3b747e0ea07e78bcae02644686309d0b7d9@oracle.com>

ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls