tls13: srv: Simplify mbedtls_ssl_read_early_data() API

Do not progress the handshake in the API, just
read early data if some has been detected by
a previous call to mbedtls_ssl_handshake(),
mbedtls_ssl_handshake_step(),
mbedtls_ssl_read() or mbedtls_ssl_write().

Signed-off-by: Ronald Cron <ronald.cron@arm.com>

library/ssl_tls13_server.c

@@ -2929,6 +2929,10 @@ static int ssl_tls13_end_of_early_data_coordinate(mbedtls_ssl_context *ssl)
          *
          * TODO: Add received data size check here.
          */
+        if (ssl->in_offt == NULL) {
+            /* Set the reading pointer */
+            ssl->in_offt = ssl->in_msg;
+        }
         return SSL_GOT_EARLY_DATA;
     }