[Ut] run topn_function_test failed in be unit test

[caiconghui]

Describe the bug

******************************
    Running Backend Unit Test    
******************************
=== Run test: topn_function_test ===
=== Run topn_function_test ===
[==========] Running 5 tests from 1 test case.
[----------] Global test environment set-up.
[----------] 5 tests from TopNFunctionsTest
[ RUN      ] TopNFunctionsTest.topn_accuracy
WARNING: Logging before InitGoogleLogging() is written to STDERR
I0303 14:12:30.217973 27712 topn_function_test.cpp:81] topn accuracy : key space : 100 , space_expand_rate : 20 , zf exponent : 0.5
ASAN:DEADLYSIGNAL
=================================================================
==27712==ERROR: AddressSanitizer: SEGV on unknown address 0xffffffffffffffe8 (pc 0x0000016d6733 bp 0x7fff5cfdabe0 sp 0x7fff5cfdaae8 T0)
==27712==The signal is caused by a READ memory access.
    #0 0x16d6732 in std::string::length() const (/home/mi/github/community/incubator-doris/be/ut_build_ASAN/test/exprs/topn_function_test+0x16d6732)
    #1 0xa2fd63 in doris::topn_single(doris_udf::FunctionContext*, std::string&, doris_udf::StringVal&, std::unordered_map<std::string, unsigned int, std::hash<std::string>, std::equal_to<std::string>, std::allocator<std::pair<std::string const, unsigned int> > >&) /home/mi/github/community/incubator-doris/be/test/exprs/topn_function_test.cpp:76
    #2 0xa30fb5 in doris::test_topn_accuracy(doris_udf::FunctionContext*, int, int, double) /home/mi/github/community/incubator-doris/be/test/exprs/topn_function_test.cpp:118
    #3 0xa33074 in doris::TopNFunctionsTest_topn_accuracy_Test::TestBody() /home/mi/github/community/incubator-doris/be/test/exprs/topn_function_test.cpp:162
    #4 0x168b19e in void testing::internal::HandleSehExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) (/home/mi/github/community/incubator-doris/be/ut_build_ASAN/test/exprs/topn_function_test+0x168b19e)
    #5 0x1686525 in void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) (/home/mi/github/community/incubator-doris/be/ut_build_ASAN/test/exprs/topn_function_test+0x1686525)
    #6 0x166d1ef in testing::Test::Run() (/home/mi/github/community/incubator-doris/be/ut_build_ASAN/test/exprs/topn_function_test+0x166d1ef)
    #7 0x166da13 in testing::TestInfo::Run() (/home/mi/github/community/incubator-doris/be/ut_build_ASAN/test/exprs/topn_function_test+0x166da13)
    #8 0x166e066 in testing::TestCase::Run() (/home/mi/github/community/incubator-doris/be/ut_build_ASAN/test/exprs/topn_function_test+0x166e066)
    #9 0x16745b2 in testing::internal::UnitTestImpl::RunAllTests() (/home/mi/github/community/incubator-doris/be/ut_build_ASAN/test/exprs/topn_function_test+0x16745b2)
    #10 0x168c03e in bool testing::internal::HandleSehExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*) (/home/mi/github/community/incubator-doris/be/ut_build_ASAN/test/exprs/topn_function_test+0x168c03e)
    #11 0x168711d in bool testing::internal::HandleExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*) (/home/mi/github/community/incubator-doris/be/ut_build_ASAN/test/exprs/topn_function_test+0x168711d)
    #12 0x167330d in testing::UnitTest::Run() (/home/mi/github/community/incubator-doris/be/ut_build_ASAN/test/exprs/topn_function_test+0x167330d)
    #13 0xa3e24c in RUN_ALL_TESTS() (/home/mi/github/community/incubator-doris/be/ut_build_ASAN/test/exprs/topn_function_test+0xa3e24c)
    #14 0xa37004 in main /home/mi/github/community/incubator-doris/be/test/exprs/topn_function_test.cpp:262
    #15 0x7fa7d7148f44 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21f44)
    #16 0x937f2e  (/home/mi/github/community/incubator-doris/be/ut_build_ASAN/test/exprs/topn_function_test+0x937f2e)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV (/home/mi/github/community/incubator-doris/be/ut_build_ASAN/test/exprs/topn_function_test+0x16d6732) in std::string::length() const
==27712==ABORTING

To Reproduce
bash run-be-ut.sh --run topn_function_test

Expected behavior
succeed in running topn_function_test
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