pack.c: fix buffer overrun

* pack.c (encodes): fix buffer overrun by tail_lf.  Thanks to
  Mamoru Tasaka and Tomas Hoger.  [ruby-core:63604] [Bug #10019]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46778 b2dd03c8-39d4-4d8f-98ff-823fe69b080e