mon/OSDMonitor: avoid null dereference if stats are not available

mon/OSDMonitor: avoid null dereference if stats are not available

Not confirmed yet whether this was the issue in the bug referenced
below, however it's a necessary defensive check for the
'osd pool get-quota' command.

All other uses of get_pool_stats() already handle this case.

Related-to: https://tracker.ceph.com/issues/53740
Signed-off-by: Josh Durgin <jdurgin@redhat.com>
(cherry picked from commit 9c8392be33a574f15bc35b8f49e319af50d99e90)

Conflicts:
	src/mon/OSDMonitor.cc

Cherry-pick notes:
- mon variable was a pointer in Octopus