Fix integer overflow; fix #3473

The fix is suggested by https://hackerone.com/lucnguyen

src/string.c

@@ -469,7 +469,7 @@ str_substr(mrb_state *mrb, mrb_value str, mrb_int beg, mrb_int len)
    beg += clen;
    if (beg < 0) return mrb_nil_value();
  }
  if (beg + len > clen)
  if (len > clen - beg)
    len = clen - beg;
  if (len <= 0) {
    len = 0;