Fix: double put on error path

Let relay_index_try_flush be responsible for the self-reference put on
error path.

Code flow of relay_index_try_flush is a bit tricky but the only error
flow (via relay_index_file_write) will always mark the index as flushed
and perform the self-reference put.

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