[Bug] BE crash when doing some queries.

[morningman]

Describe the bug
BE crash with following stack:

*** Aborted at 1592706647 (unix time) try "date -d @1592706647" if you are using GNU date ***
PC: @     0x7f05bac253f7 __GI_raise
*** SIGABRT (@0x1fa000058d5) received by PID 22741 (TID 0x7f050249a700) from PID 22741; stack trace: ***
    @     0x7f05bac25470 (unknown)
    @     0x7f05bac253f7 __GI_raise
    @     0x7f05bac267d8 __GI_abort
    @          0x26c8655 __gnu_cxx::__verbose_terminate_handler()
    @          0x262e4c6 __cxxabiv1::__terminate()
    @          0x262e889 __cxa_call_terminate
    @          0x262db78 __gxx_personality_v0
    @          0x26cfd43 _Unwind_RaiseException_Phase2
    @          0x26d087e _Unwind_Resume
    @          0x1c32cfd brpc::policy::ProcessRpcRequest()
    @          0x1c276e7 brpc::ProcessInputMessage()
    @          0x1c28561 brpc::InputMessenger::OnNewMessages()
    @          0x1cd142d brpc::Socket::ProcessEvent()
    @          0x1d83627 bthread::TaskGroup::task_runner()
    @          0x1d6f9f1 bthread_make_fcontext

To Reproduce
Just make a query with more than 1 fragments.