ext/json/parser/parser.rl: Use "signed" char to contain negative values

char is not always signed.  In fact, it is unsigned in arm.

https://rubyci.org/logs/rubyci.s3.amazonaws.com/scw-9d6766/ruby-master/log/20191004T181708Z.log.html.gz
```
compiling parser.c
parser.rl: In function ‘unescape_unicode’:
parser.rl:50:5: warning: comparison is always false due to limited range of data type [-Wtype-limits]
     if (b < 0) return UNI_REPLACEMENT_CHAR;
     ^
```

0 files changed, 0 insertions, 0 deletions