string.c: shrink before freezing

* string.c (rb_str_freeze): shrink the buffer before freezing, as
  pointed out by Eric Wong at [ruby-core:63119].

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46411 b2dd03c8-39d4-4d8f-98ff-823fe69b080e