multipart-parser: fix clang-tidy issues

```
../include/multipart_parser.hpp:77:21: error: 'buffer' is an unsafe pointer used for buffer access [-Werror,-Wunsafe-buffer-usage]
        const char* buffer = req.body().data();
../include/multipart_parser.hpp:246:38: error: 'buffer' is an unsafe pointer used for buffer access [-Werror,-Wunsafe-buffer-usage]
    void skipNonBoundary(const char* buffer, size_t len, size_t boundaryEnd,
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Iad3b4b241ec75a152e240755a307a970798079fb