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 :)
{
public:
    typedef std::reference_wrapper<const CTxMemPoolEntry> CTxMemPoolEntryRef;
    
    typedef std::set<CTxMemPoolEntryRef, CompareIteratorByHash> Parents;
    typedef std::set<CTxMemPoolEntryRef, CompareIteratorByHash> Children;
private:
    const CTransactionRef tx;
    mutable Parents m_parents;
    mutable Children m_children;
    const CAmount nFee;             
    const size_t nTxWeight;         
    const size_t nUsageSize;        
    const int64_t nTime;            
    const unsigned int entryHeight; 
    const bool spendsCoinbase;      
    const int64_t sigOpCost;        
    CAmount feeDelta{0};            
    LockPoints lockPoints;     
    
    
    
    uint64_t nCountWithDescendants{1}; 
    uint64_t nSizeWithDescendants;   
    CAmount nModFeesWithDescendants;