Fix: muxer_upstream_msg_iters_can_all_seek_beginning(): init. `status`

Fix: muxer_upstream_msg_iters_can_all_seek_beginning(): init. `status`

Detected by GCC:

    muxer.c: In function ‘muxer_msg_iter_can_seek_beginning’:
    muxer.c:1541:9: error: ‘status’ may be used uninitialized in this
    function [-Werror=maybe-uninitialized]
      return status;
             ^~~~~~
    muxer.c:1529:5: error: ‘status’ may be used uninitialized in this
    function [-Werror=maybe-uninitialized]
      if (status != BT_COMPONENT_CLASS_MESSAGE_ITERATOR_CAN_SEEK_BEGINNING_METHOD_STATUS_OK) {
         ^

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I55ad73aeb9595796bdcddff5ba4d2895d5f2314e
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2073
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>