osd/ReplicatedBackend: add sanity check during build_push_op()

osd/ReplicatedBackend: add sanity check during build_push_op()

The caller may or may not tolerate build_push_op() to return a
negative result, and we shall honour the truth that it is up
to him, not us.

Also this can silent the noise from the lower level API such as
buffer, which is less clear and can be rather confusing sometimes:

1: /usr/bin/ceph-osd() [0xada722]
2: (()+0xf100) [0x7f9889f22100]
3: (gsignal()+0x37) [0x7f988893b5f7]
4: (abort()+0x148) [0x7f988893cce8]
5: (__gnu_cxx::__verbose_terminate_handler()+0x165) [0x7f988923f9d5]
6: (()+0x5e946) [0x7f988923d946]
7: (()+0x5e973) [0x7f988923d973]
8: (()+0x5eb93) [0x7f988923db93]
9: (ceph::buffer::list::iterator::copy(unsigned int, char*)+0x137) [0xc51707]
10: (object_info_t::decode(ceph::buffer::list::iterator&)+0x7e) [0x7a236e]
11: (ReplicatedBackend::build_push_op(ObjectRecoveryInfo const&, ObjectRecoveryProgress const&, ObjectRecoveryProgress*, PushOp*, object_stat_sum_t*)+0x8dc) [0xa16c0c]
12: (ReplicatedBackend::prep_push(std::tr1::shared_ptr<ObjectContext>, hobject_t const&, pg_shard_t, eversion_t, interval_set<unsigned long>&, std::map<hobject_t, interval_set<unsigned long>, std::less<hobject_t>, std::allocator<std::pair<hobject_t const, interval_set<unsigned long> > > >&, PushOp*)+0x424) [0xa17e94]
13: (ReplicatedBackend::prep_push_to_replica(std::tr1::shared_ptr<ObjectContext>, hobject_t const&, pg_shard_t, PushOp*)+0x196) [0xa1eff6]
14: (ReplicatedBackend::start_pushes(hobject_t const&, std::tr1::shared_ptr<ObjectContext>, ReplicatedBackend::RPGHandle*)+0x1cd) [0xa1fcad]
15: (C_ReplicatedBackend_OnPullComplete::finish(ThreadPool::TPHandle&)+0x153) [0xa32173]
16: (GenContext<ThreadPool::TPHandle&>::complete(ThreadPool::TPHandle&)+0x9) [0x6c5ea9]
17: (ReplicatedPG::BlessedGenContext<ThreadPool::TPHandle&>::finish(ThreadPool::TPHandle&)+0x95) [0x8acbe5]
18: (GenContext<ThreadPool::TPHandle&>::complete(ThreadPool::TPHandle&)+0x9) [0x6c5ea9]
19: (ThreadPool::WorkQueueVal<GenContext<ThreadPool::TPHandle&>*, GenContext<ThreadPool::TPHandle&>*>::_void_process(void*, ThreadPool::TPHandle&)+0x62) [0x6c7f22]
20: (ThreadPool::worker(ThreadPool::WorkThread*)+0xa76) [0xbce296]
21: (ThreadPool::WorkThread::entry()+0x10) [0xbcf320]
22: (()+0x7dc5) [0x7f9889f1adc5]
23: (clone()+0x6d) [0x7f98889fc21d]

Signed-off-by: Yan Jun <yan.jun8@zte.com.cn>