src/mon/OSDMonitor.cc: ignore non-exist CRUSH bucket

src/mon/OSDMonitor.cc: ignore non-exist CRUSH bucket

Problem:

In a stretch cluster, we encountered
an assert failure when checking for
dead crush zones when we have a none-existing
CRUSH bucket.

Solution:

Ignore the none-existing crush bucket, instead
of assert.

Fixes: https://tracker.ceph.com/issues/63861
Signed-off-by: Kamoltat Sirivadhna <ksirivad@redhat.com>