mon/OSDMonitor: Added extra check before mon.go_recovery_stretch_mode()

mon/OSDMonitor: Added extra check before mon.go_recovery_stretch_mode()

Problem:
There are certain scenarios in degraded
stretched cluster where will try to
go into the
function ``Monitor::go_recovery_stretch_mode()``
that will lead to a `ceph_assert`.

Solution:
Make sure ``dead_mon_buckets.size() == 0``
in ``OSDMonitor:update_from_paxos()``
before going into ``Monitor::go_recovery_stretch_mode()``.

Fixes:
https://bugzilla.redhat.com/show_bug.cgi?id=2104207

Signed-off-by: Kamoltat <ksirivad@redhat.com>