* lib/fileutils.rb (FileUtils::Entry_#copy): ensure that the
  source entry exists first, to fix error for non-existent entry.
  [ruby-core:77885] [Bug #12892]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56551 b2dd03c8-39d4-4d8f-98ff-823fe69b080e