mon: OSDMonitor: don't crash if formatter is invalid during osd crush dump

Code would assume a formatter would always be defined. If a 'plain'
formatter or even an invalid formatter were to be supplied, the monitor
would crash and burn in poor style.

Fixes: 6820
Backport: emperor

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
(cherry picked from commit 49d2fb71422fe4edfe5795c001104fb5bc8c98c3)
Merge pull request #1366 from ceph/wip-6820.dumpling

mon: OSDMonitor: don't crash if formatter is invalid during osd crush dump