I noticed this bug while working on something else with
RUBYOPT=-d on, existing test cases all passed with it.

Note: I use String.new because it is the local style, here,
I prefer +'' (or ''.b, for a future commit)

* lib/net/ftp.rb (BufferedSocket#read): use String.new
* test/net/ftp/test_buffered_socket.rb (test_read_nil): new test
  [ruby-core:84675] [Bug #14323]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61639 b2dd03c8-39d4-4d8f-98ff-823fe69b080e