service: ensure the ipsec path is NUL terminated

[lkundrak]

GCC 8.2 complains:

src/nm-l2tp-service.c: In function ‘real_connect’: src/nm-l2tp-service.c:1370:3: warning: ‘strncpy’ specified bound 256 equals destination size [-Wstringop-truncation] strncpy (priv->ipsec_binary_path, value, sizeof(priv->ipsec_binary_path)); ^~~~~~~~~~~~~~~~~