Signed integer overflow when SipHasher processes inputs >= 2 GB.

Live demo:

```
$ src/test/fuzz/simplest_possible_siphash_fuzzer -rss_limit_mb=8000 crash-061a172add013c03beedf57eb2a121a8289696af
crypto/siphash.cpp:56:10: runtime error: signed integer overflow: 2147483647 + 1 cannot be represented in type 'int'
$ cat src/test/fuzz/simplest_possible_siphash_fuzzer.cpp
#include <cstdint>
#include <vector>

#include <crypto/siphash.h>

void test_one_input(const std::vector<uint8_t>& buffer)
{
    CSipHasher(0, 0).Write(buffer.data(), buffer.size()).Finalize();
}
```

Credits to @elichai who submitted a differential SipHasher fuzzer in #19920 and @guidovranken who first spotted the issue. Thanks!

**Remember: don't trust -- fuzz! :)**
#ifndef BITCOIN_CRYPTO_SIPHASH_H
#define BITCOIN_CRYPTO_SIPHASH_H
#include <stdint.h>
#include <uint256.h>
class CSipHasher
{
private: