meta crash when drop space if meta leader change

[nebula-bots]

1. meta leader change
2. drop a hosts

expect result:

1. return leader change error

actual result:

1. meta crash

(gdb) bt
#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
#1  0x00007fb0584b5859 in __GI_abort () at abort.c:79
#2  0x0000000002d755ba in ?? ()
#3  0x0000000002d786b4 in ?? ()
#4  0x0000000002d75089 in ?? ()
#5  0x0000000002d78d69 in ?? ()
#6  0x0000000001484d9d in nebula::EitherOr<nebula::cpp2::ErrorCode, std::unique_ptr<nebula::kvstore::KVIterator, std::default_delete<nebula::kvstore::KVIterator> > >::right() & ()
#7  0x00000000015c1533 in nebula::meta::DropHostsProcessor::process(nebula::meta::cpp2::DropHostsReq const&) ()
#8  0x00000000014495b0 in nebula::meta::MetaServiceHandler::future_dropHosts(nebula::meta::cpp2::DropHostsReq const&) ()
#9  0x0000000001b5f1b8 in nebula::meta::cpp2::MetaServiceSvIf::async_tm_dropHosts(std::unique_ptr<apache::thrift::HandlerCallback<nebula::meta::cpp2::ExecResp>, std::default_delete<apache::thrift::HandlerCallback<nebula::meta::cpp2::ExecResp> > >, nebula::meta::cpp2::DropHostsReq const&) ()
#10 0x0000000001bf1205 in void nebula::meta::cpp2::MetaServiceAsyncProcessor::process_dropHosts<apache::thrift::CompactProtocolReader, apache::thrift::CompactProtocolWriter>(std::unique_ptr<apache::thrift::ResponseChannelRequest, apache::thrift::RequestsRegistry::Deleter>, apache::thrift::SerializedCompressedRequest&&, apache::thrift::Cpp2RequestContext*, folly::EventBase*, apache::thrift::concurrency::ThreadManager*) ()
#11 0x0000000001b740e3 in apache::thrift::RequestTask<nebula::meta::cpp2::MetaServiceAsyncProcessor>::run() ()
#12 0x000000000282c6e7 in virtual thunk to apache::thrift::concurrency::FunctionRunner::run() ()
#13 0x0000000002988dd8 in apache::thrift::concurrency::ThreadManager::Impl::Worker::run() ()
#14 0x000000000298aede in apache::thrift::concurrency::PthreadThread::threadMain(void*) ()
#15 0x00007fb05868d609 in start_thread (arg=<optimized out>) at pthread_create.c:477
#16 0x00007fb0585b2133 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95