rbd-mirror: fix segfault in snapshot replayer shutdown

rbd-mirror: fix segfault in snapshot replayer shutdown

If an error arises in the init flow of the snapshot replayer and the
function returns before the call on `register_local_update_watcher`
the value of `m_update_watch_ctx` will not be initialized. Therefore,
on the shutdown phase, the replayer will try to free this pointer
and segfault.

This commit fixes this issue by setting `m_update_watch_ctx` to
`nullptr`.

Fixes: https://tracker.ceph.com/issues/50931
Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@cern.ch>