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 :)
uint64_t limitAncestorCount,
                                    uint64_t limitAncestorSize,
                                    uint64_t limitDescendantCount,
                                    uint64_t limitDescendantSize,
                                    std::string &errString) const
{
    CTxMemPoolEntry::Parents staged_ancestors;
    size_t total_size = 0;
    for (const auto& tx : package) {
        total_size += GetVirtualTransactionSize(*tx);
        for (const auto& input : tx->vin) {
            std::optional<txiter> piter = GetIter(input.prevout.hash);
            if (piter) {
                staged_ancestors.insert(**piter);
                if (staged_ancestors.size() + package.size() > limitAncestorCount) {
                    errString = strprintf("too many unconfirmed parents [limit: %u]", limitAncestorCount);
                    return false;
                }
            }
        }
    }