mon: Paxos: only finish a queued proposal if there's actually *any*

When proposing an older value learned during recovery, we don't create
a queued proposal -- we go straight through Paxos.  Therefore, when
finishing a proposal, we must be sure that we have a proposal in the queue
before dereferencing it, otherwise we will segfault.

Fixes: #4250

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>