Fix: src.ctf.fs: init_index_entry() may return uninitialized value

Fix: src.ctf.fs: init_index_entry() may return uninitialized value

Found with scan-build.
Warning reported: Undefined or garbage value returned to caller

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Ifaccdd9f7c8a762fd4eb9f26a5fce4cd092c44df
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1259
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins