Fix http_parser check for architectures with unsigned chars

http_parser.c has a table named "unhex" that it uses to convert hex
characters to their numeric values, e.g. 'F' -> 15. For non-hex
characters, the value is -1 but while the table contains int8_t values,
the extraction is done using a char. On ARMv8, char is *unsigned*, which
means it can't be compared to -1 as this is always false. Comparing to
(char)-1 instead will work.

src/http-parser/http_parser.c

@@ -1440,7 +1440,7 @@ size_t http_parser_execute (http_parser *parser,
         assert(parser->flags & F_CHUNKED);
 
         c = unhex[(unsigned char)ch];
-        if (c == -1) goto error;
+        if (c == (char)-1) goto error;
         parser->content_length = c;
         state = s_chunk_size;
         break;
@@ -1457,7 +1457,7 @@ size_t http_parser_execute (http_parser *parser,
 
         c = unhex[(unsigned char)ch];
 
-        if (c == -1) {
+        if (c == (char)-1) {
           if (ch == ';' || ch == ' ') {
             state = s_chunk_parameters;
             break;