Fix "array subscript is of type 'char'" on NetBSD 9

In file included from rdata.c:602:
In file included from ./code.h:88:
./rdata/in_1/svcb_64.c:259:9: warning: array subscript is of type 'char' [-Wchar-subscripts]
                        if (!isdigit(*region->base)) {
                             ^~~~~~~~~~~~~~~~~~~~~~
/usr/include/sys/ctype_inline.h:51:44: note: expanded from macro 'isdigit'
#define isdigit(c)      ((int)((_ctype_tab_ + 1)[(c)] & _CTYPE_D))
                                                ^~~~