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;