BE crashed when using Broker Load to import tables containing bitmap BITMAP_UNION

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Describe the bug
BE crashed when using Broker Load to import tables containing bitmap BITMAP_UNION

I used the latest master complie.

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*** Aborted at 1591843669 (unix time) try "date -d @1591843669" if you are using GNU date *** PC: @ 0xea4411 doris::AggregateFuncTraits<>::init() *** SIGSEGV (@0x1b50a5c0) received by PID 26620 (TID 0x7f017dce5700) from PID 458270144; stack trace: *** @ 0x7f01ff70d340 (unknown) @ 0xea4411 doris::AggregateFuncTraits<>::init() @ 0xecf3ff doris::MemTable::insert() @ 0x156f946 doris::DeltaWriter::write() @ 0xff51f6 doris::TabletsChannel::add_batch() @ 0xff1a46 doris::LoadChannel::add_batch() @ 0xfed041 doris::LoadChannelMgr::add_batch() @ 0x1071a55 _ZZN5doris20PInternalServiceImplIN4palo16PInternalServiceEE23tablet_writer_add_batchEPN6google8protobuf13RpcControllerEPKNS_28PTabletWriterAddBatchRequestEPNS_27PTabletWriterAddBatchResultEPNS5_7ClosureEENKUlvE_clEv @ 0xf869a5 doris::PriorityThreadPool::work_thread() @ 0x1a37aad thread_proxy @ 0x7f01ff4c2dd5 start_thread @ 0x7f01ff7d502d __clone