os/FileStore::OpSequencer: operator<<: silence gcc warning

os/FileStore::OpSequencer: operator<<: silence gcc warning

it kills following warning:
```
/home/kefu/dev/ceph/src/os/filestore/FileStore.cc: In function
‘std::ostream& operator<<(std::ostream&, const
FileStore::OpSequencer&)’:
/home/kefu/dev/ceph/src/os/filestore/FileStore.cc:155:132: warning: the
compiler can assume that the address of ‘out’ will always evaluate to
‘true’ [-Waddress]
   assert(&out);
```

Signed-off-by: Kefu Chai <kchai@redhat.com>