messages/MOSDPing: fix the inflation amount calculation

messages/MOSDPing: fix the inflation amount calculation

If user specifies a min_message_size small enough (or zero to disable
it altogether), OSDs will crash and burn while trying to allocate
almost 4GB of payload (both min_message_size and payload.length() are
unsigned, so it'll roll over back to 4GB and MAX(4GB, 0) will use 4GB).
If the size of dummy payload is 0, don't bother constructing bufferptr
and bufferlist, then encoding that.

Signed-off-by: Piotr Dałek <piotr.dalek@corp.ovh.com>