Fix: src.ctf.fs: initialize the other_entry variable

Fix: src.ctf.fs: initialize the other_entry variable

Initialize the pointer other_entry to silence this warning:

    | ../../../../../git/src/plugins/ctf/fs-src/fs.c: In function 'ds_index_insert_ds_index_entry_sorted':
    | ../../../../../git/src/plugins/ctf/fs-src/fs.c:702:5: error: 'other_entry' may be used uninitialized in this function [-Werror=maybe-uninitialized]
    |  702 |    !ds_index_entries_equal(entry, other_entry)) {

This was encountered with gcc 9.2.0 at the -Og optimization level.
After inspection, it appears that this is a false positive, the
`other_entry` pointer can only be used after being initialized first.

Signed-off-by: Mingli Yu <mingli.yu@windriver.com>
Change-Id: Icf63e605cf543c3eb29e5aadec18b22b137ee9da
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3353
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>