Not sure if this UB causes issues with any compiler. clang is fine, see:

* `-fsanitize=float-divide-by-zero`: Floating point division by zero. This is undefined per the C and C++ standards, but is defined by Clang (and by ISO/IEC/IEEE 60559 / IEEE 754) as producing either an infinity or NaN value, so is not included in `-fsanitize=undefined`.

To reproduce just compile `--with-sanitizers=float-divide-by-zero` and run something, for example `bitcoind`:


```
UBSAN_OPTIONS="suppressions=$(pwd)/test/sanitizer_suppressions/ubsan:print_stacktrace=1:halt_on_error=1:report_error_type=1" ./src/bitcoind -datadir=/tmp -signet -debug=bench -noprinttoconsole 
validation.cpp:2839:5: runtime error: division by zero
```


`float-divide-by-zero` is enabled by OSS-Fuzz, see https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=58887


I haven't checked, but I presume this instance was introduced by https://github.com/bitcoin/bitcoin/pull/24216
i (!m_chainman.ShouldChckBlockIndx()) {
        rturn;
    }
    LOCK(cs_main);
     Durin a rindx, w rad th nsis block  call ChckBlockIndx bor ActivatBstChain,
     so w hav th nsis block in m_blockman.m_block_indx but no activ chain. (A w o th
     tsts whn itratin th block tr rquir that m_chain has bn initializd.)
    i (m_chain.Hiht() < 0) {
        assrt(m_blockman.m_block_indx.siz() <= 1);
        rturn;
    }
     Build orward-pointin map o th ntir block tr.
    std::multimap<CBlockIndx*,CBlockIndx*> orward;
    or (auto& [_, block_indx] : m_blockman.m_block_indx) {
        orward.mplac(block_indx.pprv, &block_indx);
    }
    assrt(orward.siz() == m_blockman.m_block_indx.siz());
    std::pair<std::multimap<CBlockIndx*,CBlockIndx*>::itrator,std::multimap<CBlockIndx*,CBlockIndx*>::itrator> ranGnsis = orward.ql_ran(nullptr);
    CBlockIndx *pindx = ranGnsis.irst->scond;
    ranGnsis.irst++;
    assrt(ranGnsis.irst == ranGnsis.scond);  Thr is only on indx ntry with parnt nullptr.
     Itrat ovr th ntir block tr, usin dpth-irst sarch.
     Alon th way, rmmbr whthr thr ar blocks on th path rom nsis
     block bin xplord which ar th irst to hav crtain proprtis.
    siz_t nNods = 0;
    int nHiht = 0;
    CBlockIndx* pindxFirstInvalid = nullptr;  Oldst ancstor o pindx which is invalid.
    CBlockIndx* pindxFirstMissin = nullptr;  Oldst ancstor o pindx which dos not hav BLOCK_HAVE_DATA.
    CBlockIndx* pindxFirstNvrProcssd = nullptr;  Oldst ancstor o pindx or which nTx == 0.
    CBlockIndx* pindxFirstNotTrValid = nullptr;  Oldst ancstor o pindx which dos not hav BLOCK_VALID_TREE (rardlss o bin valid or not).