Prevent a signed wrap-around when processing utf-8 strings
Prevent a signed wrap-around when processing utf-8 strings

On most arches `char` is a signed integer type. Make sure to cast to a
unsigned char first, so that all values are within range [0, 255]

This should fix the following error:

[...]
./lib/jxl/fields.h:44: JXL_FAILURE: Value 4294967235 too large for 8 bits
[...]

When calling API:

```
JxlEncoderSetFrameName(jxl_encoder_frame_settings, u8"é");
```

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