**Is your feature request related to a problem? Please describe.**

Warning on building using g++ 8.3.

```
 1 In file included from ./txdb.h:11,
 2                  from ./validation.h:26,
 3                  from ./test/util/chainstate.h:13,
 4                  from test/validation_chainstate_tests.cpp:11:
 5 ./chain.h: In member function ‘void validation_chainstate_tests::chainstate_update_tip::test_method()’:
 6 ./chain.h:422:27: warning: ‘background_cs’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 7          return vChain.size() > 0 ? vChain[vChain.size() - 1] : nullptr;
 8                 ~~~~~~~~~~~^~
 9 test/validation_chainstate_tests.cpp:109:18: note: ‘background_cs’ was declared here
10      CChainState* background_cs;
11                   ^~~~~~~~~~~~~
12
```

**Describe the solution you'd like**

Able to build without warnings.

**Describe alternatives you've considered**

N/A.

**Additional context**

`cd src/ && make test/test_bitcoin-validation_chainstate_tests.o`

Bisecting this warning, I ended up in revision 673a5bd3377929a0
by @jamesob.

Thank you very much and please excuse the noise.

std::vector<CMutableTransaction> noTxns;
    CScript scriptPubKey = CScript() << ToByteVector(coinbaseKey.GetPubKey()) << OP_CHECKSIG;
    CBlock validation_block = this->CreateBlock(noTxns, scriptPubKey, *background_cs);
    auto pblock = std::make_shared<const CBlock>(validation_block);
    BlockValidationState state;
    CBlockIndex* pindex = nullptr;
    const CChainParams& chainparams = Params();
    bool newblock = false;
    
    
    {
        LOCK(::cs_main);
        bool checked = CheckBlock(*pblock, state, chainparams.GetConsensus());
        BOOST_CHECK(checked);
        bool accepted = background_cs->AcceptBlock(
            pblock, state, &pindex, true, nullptr, &newblock);
        BOOST_CHECK(accepted);
    }
    
    bool block_added = background_cs->ActivateBestChain(state, pblock);
    
    BOOST_CHECK_EQUAL(background_cs->m_chain.Tip()->GetBlockHash(), validation_block.GetHash());
    
    
    BOOST_CHECK(block_added);
    BOOST_CHECK_EQUAL(curr_tip, ::g_best_block);
}
BOOST_AUTO_TEST_SUITE_END()