os/bluestore: do not segv on kraken upgrade debug print

os/bluestore: do not segv on kraken upgrade debug print

When loading an onode from kraken we have a compat path that calls
get_ref before the SharedBlob pointer is initialized.  This is fine except
that if debugging is enabled the operator<< on the Blob will segv on
printing *b.shared_blob (which is NULL).

Fix operator<< to print something else if it is NULL.  shared_blob does
get set up right after the call to decode() so having it be NULL at this
point is otherwise harmless.

Fixes: http://tracker.ceph.com/issues/20977
Signed-off-by: Sage Weil <sage@redhat.com>