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 :)
EXPIRY,      
    SIZELIMIT,   
    REORG,       
    BLOCK,       
    CONFLICT,    
    REPLACED,    
};
/**
 * CTxMemPool stores valid-according-to-the-current-best-chain transactions
 * that may be included in the next block.
 *
 * Transactions are added when they are seen on the network (or created by the
 * local node), but not all transactions seen are added to the pool. For
 * example, the following new transactions will not be added to the mempool:
 * - a transaction which doesn't meet the minimum fee requirements.
 * - a new transaction that double-spends an input of a transaction already in
 * the pool where the new transaction does not meet the Replace-By-Fee
 * requirements as defined in BIP 125.
 * - a non-standard transaction.
 *
 * CTxMemPool::mapTx, and CTxMemPoolEntry bookkeeping: