[core] fix abort in http-parseopts (fixes #2945)

fix abort in server.http-parseopts with url-path-2f-decode enabled

(thx stze)

x-ref:
  "Security - SIGABRT during GET request handling with url-path-2f-decode enabled"
  https://redmine.lighttpd.net/issues/2945
static void burl_offset_tolower (buffer * const b, const size_t off)
{
    /*(skips over all percent-encodings, including encoding of alpha chars)*/
    for (char *p = b->ptr+off; p[0]; ++p) {
        if (p[0] >= 'A' && p[0] <= 'Z') p[0] |= 0x20;
        else if (p[0]=='%' && light_isxdigit(p[1]) && light_isxdigit(p[2]))
            p+=2;
    }
}

static void burl_offset_tolower (buffer * const b, const size_t off)
{
    /*(skips over all percent-encodings, including encoding of alpha chars)*/
    for (char *p = b->ptr+off; p[0]; ++p) {
        if (p[0] >= 'A' && p[0] <= 'Z') p[0] |= 0x20;
        else if (p[0]=='%' && light_isxdigit(p[1]) && light_isxdigit(p[2]))
            p+=2;
    }
}
