Fix: potential use of uninitialized return value

lttng_tracker_ids_serialize() can return an uninitialized value
when 0 ids are being tracked by a tracker. This is not currently
reachable, but generates a warning on some compilers.

'ret' may be used uninitialized in this function [-Wmaybe-uninitialized]

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ia54f5276d6a89d39badfc6c718ad0032edb98ec8