Assign *_STATUS_OK to ret

Assign *_STATUS_OK to ret

pretty.c:227 'ret' may be used uninitialized in this function [-Wmaybe-uninitialized]

This *could* happen if next_status is BT_MESSAGE_ITERATOR_NEXT_STATUS_OK
and count is still 0.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Change-Id: I4a08d1bcc2f9b56e225ca17a210a4597169ed746
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1634
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>