cephfs_mirror: check m_instance_watcher and m_mirror_watcher before using them

cephfs_mirror: check m_instance_watcher and m_mirror_watcher before using them

Fixes the crash:
```
std::scoped_lock<std::mutex>::scoped_lock (__m=..., this=<optimized out>, this=<optimized out>, __m=...) at /usr/include/c++/11/mutex:655
cephfs::mirror::MirrorWatcher::is_failed (this=0x0) at /usr/src/debug/ceph-19.0.0-1578.g3d482f42.el9.x86_64/src/tools/cephfs_mirror/MirrorWatcher.h:56
cephfs::mirror::FSMirror::is_failed (this=0x557478284340) at /usr/src/debug/ceph-19.0.0-1578.g3d482f42.el9.x86_64/src/tools/cephfs_mirror/FSMirror.h:52
cephfs::mirror::Mirror::update_fs_mirrors (this=0x5574773c3e60) at /usr/src/debug/ceph-19.0.0-1578.g3d482f42.el9.x86_64/src/tools/cephfs_mirror/Mirror.cc:515

```

Fixes: https://tracker.ceph.com/issues/64751
Signed-off-by: Jos Collin <jcollin@redhat.com>