Visual Studio 2019 overflow warning

Building Hyperscan 5.2.1 in Visual Studio 2019 results in a buffer overflow warning:
```
hyperscan\src\util\simd_utils.h(613): warning C4789: buffer 'tempbuf' of size 24 bytes will be overrun; 16 bytes will be written starting at offset 16
```

The [code in question](https://github.com/intel/hyperscan/blob/v5.2.1/src/util/simd_utils.h#L613) is the non-AVX2 branch of `storeu256`.