Fix mbedtls_ssl_get_version() for TLSv1.3

Test it in ssl-opt.sh.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

tests/ssl-opt.sh

@@ -9668,6 +9668,7 @@ run_test    "TLS 1.3: minimal feature sets - openssl" \
             -c "<= parse certificate verify"          \
             -c "mbedtls_ssl_tls13_process_certificate_verify() returned 0" \
             -c "<= parse finished message" \
+            -c "Protocol is TLSv1.3" \
             -c "HTTP/1.0 200 ok"
 
 requires_gnutls_tls1_3