file.c: fix buffer overflow

* file.c (rb_readlink): fix buffer overflow on a long symlink. since
  rb_str_modify_expand() expands from its length but not its capacity,
  need to set the length properly for each expansion.
  [ruby-core:58592] [Bug #9157]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43853 b2dd03c8-39d4-4d8f-98ff-823fe69b080e