Fix: initialize temporary fd array element to -1

Fix: initialize temporary fd array element to -1

These are FDs, an initialization value of zero is a valid FD number
hence we must initialize the element to -1.

This allows us to be more strict in `add_fds_to_payload` and assert that
the passed FDs are actual FDs and not only bare zero-initialized
element.

Change-Id: Icc05d98d51906636362a67588d7c42b58cf33ded
Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>