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 :)
{
protected:
    const int m_check_ratio; 
    std::atomic<unsigned int> nTransactionsUpdated{0}; 
    CBlockPolicyEstimator* const minerPolicyEstimator;
    uint64_t totalTxSize GUARDED_BY(cs);      
    CAmount m_total_fee GUARDED_BY(cs);       
    uint64_t cachedInnerUsage GUARDED_BY(cs); 
    mutable int64_t lastRollingFeeUpdate GUARDED_BY(cs);
    mutable bool blockSinceLastRollingFeeBump GUARDED_BY(cs);
    mutable double rollingMinimumFeeRate GUARDED_BY(cs); 
    mutable Epoch m_epoch GUARDED_BY(cs);
    
    
    
    mutable uint64_t m_sequence_number GUARDED_BY(cs){1};
    void trackPackageRemoved(const CFeeRate& rate) EXCLUSIVE_LOCKS_REQUIRED(cs);
    bool m_is_loaded GUARDED_BY(cs){false};
public:
    static const int ROLLING_FEE_HALFLIFE = 60 * 60 * 12; 
    typedef boost::multi_index_container<
        CTxMemPoolEntry,
        boost::multi_index::indexed_by<
            
            boost::multi_index::hashed_unique<mempoolentry_txid, SaltedTxidHasher>,