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 :)
{
    int64_t updateCount = setAncestors.size();
    int64_t updateSize = 0;
    CAmount updateFee = 0;
    int64_t updateSigOpsCost = 0;
    for (txiter ancestorIt : setAncestors) {
        updateSize += ancestorIt->GetTxSize();
        updateFee += ancestorIt->GetModifiedFee();
        updateSigOpsCost += ancestorIt->GetSigOpCost();
    }
    mapTx.modify(it, update_ancestor_state(updateSize, updateFee, updateCount, updateSigOpsCost));
}
void CTxMemPool::UpdateChildrenForRemoval(txiter it)
{
    const CTxMemPoolEntry::Children& children = it->GetMemPoolChildrenConst();
    for (const CTxMemPoolEntry& updateIt : children) {
        UpdateParent(mapTx.iterator_to(updateIt), it, false);
    }
}
void CTxMemPool::UpdateForRemoveFromMempool(const setEntries &entriesToRemove, bool updateDescendants)