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))

(cherry picked from commit d0944728)
79 jobs for !5594 with mnowak/warning-array-subscript-is-of-type-char-on-netbsd-9-v9_16 in 18 minutes and 56 seconds (queued for 14 seconds)
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