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 :)
{
    for (unsigned int i = 0; i < tx.vin.size(); i++)
        if (exists(GenTxid::Txid(tx.vin[i].prevout.hash)))
            return false;
    return true;
}
CCoinsViewMemPool::CCoinsViewMemPool(CCoinsView* baseIn, const CTxMemPool& mempoolIn) : CCoinsViewBacked(baseIn), mempool(mempoolIn) { }
bool CCoinsViewMemPool::GetCoin(const COutPoint &outpoint, Coin &coin) const {
    
    
    if (auto it = m_temp_added.find(outpoint); it != m_temp_added.end()) {
        coin = it->second;
        return true;
    }
    
    
    
    CTransactionRef ptx = mempool.get(outpoint.hash);
    if (ptx) {
        if (outpoint.n < ptx->vout.size()) {
            coin = Coin(ptx->vout[outpoint.n], MEMPOOL_HEIGHT, false);
            return true;
        } else {
            return false;
        }
    }
    return base->GetCoin(outpoint, coin);
}
void CCoinsViewMemPool::PackageAddTransaction(const CTransactionRef& tx)