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;
     ^
```

Author: mame

ext/json/ext/parser/parser.c

@@ -27,7 +27,7 @@ enc_raise(rb_encoding *enc, VALUE exc, const char *fmt, ...)
 
 /* unicode */
 
-static const char digit_values[256] = {
+static const signed char digit_values[256] = {
     -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
     -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
     -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, -1,
@@ -46,7 +46,7 @@ static const char digit_values[256] = {
 
 static UTF32 unescape_unicode(const unsigned char *p)
 {
-    char b;
+    signed char b;
     UTF32 result = 0;
     b = digit_values[p[0]];
     if (b < 0) return UNI_REPLACEMENT_CHAR;

ext/json/ext/parser/parser.rl

@@ -25,7 +25,7 @@ enc_raise(rb_encoding *enc, VALUE exc, const char *fmt, ...)
 
 /* unicode */
 
-static const char digit_values[256] = {
+static const signed char digit_values[256] = {
     -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
     -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
     -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, -1,
@@ -44,7 +44,7 @@ static const char digit_values[256] = {
 
 static UTF32 unescape_unicode(const unsigned char *p)
 {
-    char b;
+    signed char b;
     UTF32 result = 0;
     b = digit_values[p[0]];
     if (b < 0) return UNI_REPLACEMENT_CHAR;