net/ftp: fix FrozenError in BufferedSocket

	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
	  [Bug #14323]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_3@62943 b2dd03c8-39d4-4d8f-98ff-823fe69b080e