While fuzzing the RPC interface I stumbled upon a signed integer overflow in `CTxMemPool::PrioritiseTransaction(…)` which is reachable via the following two `prioritisetransaction` RPC calls:

```
$ ./autogen.sh
$ CC=clang CXX=clang++ ./configure --with-sanitizers=address,undefined
$ make
$ UBSAN_OPTIONS="print_stacktrace=1" src/bitcoind &
$ src/bitcoin-cli prioritisetransaction cafebabecafebabecafebabecafebabecafebabecafebabecafebabecafebabe 0 -9123456789123456789
$ src/bitcoin-cli prioritisetransaction cafebabecafebabecafebabecafebabecafebabecafebabecafebabecafebabe 0 -9123456789123456789
txmempool.cpp:832:15: runtime error: signed integer overflow: -9123456789123456789 + -9123456789123456789 cannot be represented in type 'long'
    #0 0x5581f3e69c3c in CTxMemPool::PrioritiseTransaction(uint256 const&, long const&) src/txmempool.cpp:832:15
    #1 0x5581f3c93852 in prioritisetransaction()::$_6::operator()(RPCHelpMan const&, JSONRPCRequest const&) const src/rpc/mining.cpp:470:36
…

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior txmempool.cpp:832:15
```

Nothing high priority of course, but still worth fixing :)
LOCK(cs);
    auto it = mapTx.find(txid);
    ancestors = descendants = 0;
    if (it != mapTx.end()) {
        ancestors = it->GetCountWithAncestors();
        if (ancestorsize) *ancestorsize = it->GetSizeWithAncestors();
        if (ancestorfees) *ancestorfees = it->GetModFeesWithAncestors();
        descendants = CalculateDescendantMaximum(it);
    }
}
bool CTxMemPool::IsLoaded() const
{
    LOCK(cs);
    return m_is_loaded;
}
void CTxMemPool::SetIsLoaded(bool loaded)
{
    LOCK(cs);
    m_is_loaded = loaded;
}