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.
Detected by running compat.sh in config-ccm-psk-dtls1_2.h.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>