string.c: fix one-off bug

* string.c (rb_str_cat_conv_enc_opts): fix one-off bug.  `ofs`
  equals `olen` when appending at the end.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58702 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

string.c

@@ -917,7 +917,7 @@ rb_str_cat_conv_enc_opts(VALUE newstr, long ofs, const char *ptr, long len,
     long olen;
 
     olen = RSTRING_LEN(newstr);
-    if (ofs < -olen || olen <= ofs)
+    if (ofs < -olen || olen < ofs)
         rb_raise(rb_eIndexError, "index %ld out of string", ofs);
     if (ofs < 0) ofs += olen;
     if (!from) {