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 :)
{
        return a.get().GetTx().GetHash() < b.get().GetTx().GetHash();
    }
    template <typename T>
    bool operator()(const T& a, const T& b) const
    {
        return a->GetTx().GetHash() < b->GetTx().GetHash();
    }
};
/** \class CTxMemPoolEntry
 *
 * CTxMemPoolEntry stores data about the corresponding transaction, as well
 * as data about all in-mempool transactions that depend on the transaction
 * ("descendant" transactions).
 *
 * When a new entry is added to the mempool, we update the descendant state
 * (nCountWithDescendants, nSizeWithDescendants, and nModFeesWithDescendants) for
 * all ancestors of the newly added transaction.
 *
 */
class CTxMemPoolEntry