Undersized SipHash key leads to buffer out-of-bounds read

```cpp
#include <seckey.h>
#include <siphash.h>

int main(void)
{
    const uint8_t key[3] = { 0 };
    ::CryptoPP::SipHash<2, 4, false> siphash(key, sizeof(key));
    return 0;
}
```

I think it would be more appropriate to throw an exception in this case?