Fix mbedtls_ssl_get_version() for TLSv1.3

Test it in ssl-opt.sh.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index dd05716..0548c14 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -9668,6 +9668,7 @@
             -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