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 :)
cachedInnerUsage += entry.DynamicMemoryUsage();
    const CTransaction& tx = newit->GetTx();
    std::set<uint256> setParentTransactions;
    for (unsigned int i = 0; i < tx.vin.size(); i++) {
        mapNextTx.insert(std::make_pair(&tx.vin[i].prevout, &tx));
        setParentTransactions.insert(tx.vin[i].prevout.hash);
    }
    
    
    
    
    
    
    
    for (const auto& pit : GetIterSet(setParentTransactions)) {
            UpdateParent(newit, pit, true);
    }
    UpdateAncestorsOf(true, newit, setAncestors);
    UpdateEntryForAncestors(newit, setAncestors);
    nTransactionsUpdated++;
    totalTxSize += entry.GetTxSize();
    m_total_fee += entry.GetFee();
    if (minerPolicyEstimator) {
        minerPolicyEstimator->processTransaction(entry, validFeeEstimate);
    }
    vTxHashes.emplace_back(tx.GetWitnessHash(), newit);
    newit->vTxHashesIdx = vTxHashes.size() - 1;
}
void CTxMemPool::removeUnchecked(txiter it, MemPoolRemovalReason reason)