Silence strncpy warning emitted by GCC 8 in ini parser

While copying 'dst len' bytes in strncpy is normally risky
as the dst may not be NULL-terminated, this function ensures
that the last byte of 'dst' is NULL.

Therefore, this change is mostly made to silence GCC.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>