* string.c (rb_fstring, fstr_update_callback): simply delete garbage
  key first.
  Garbage keys can be swept by lazy sweeping invoked by creating new
  fstring.  So that simply do:
  (1) delete garbage key and return `fstr_update_callback' immediately
  (2) try again `fstr_update_callback()' to create a new fstr.
  This bug can be cause memory corruption, reported by
  http://u64.rubyci.org/~chkbuild/ruby-trunk/log/20140821T220302Z.fail.html.gz



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47252 b2dd03c8-39d4-4d8f-98ff-823fe69b080e