[ruby/reline] Fix Reline::Unicode.calculate_width when input is not a TTY

This fixes an error when output is redirected:

```
$ run_ruby -rreline -e '$stderr.puts Reline::Unicode.calculate_width("\u221a").inspect' </dev/null >/dev/null
/home/jeremy/tmp/ruby/lib/reline/ansi.rb:189:in `raw': Operation not supported by device (Errno::ENODEV)
```

The @@encoding -> defined?(@@encoding) changes is necessary because
without that part of the commit, the following error would be raised
by the above command:

```
/home/jeremy/tmp/reline/lib/reline/general_io.rb:10:in `encoding': uninitialized class variable @@encoding in Reline::GeneralIO (NameError)
```

Problem reported and initial patch for Windows provided by
Richard Sharman.

I tested this only on OpenBSD, but hopefully it works for other
operating systems.

Fixes [Bug #17493]

c001971bb3

lib/reline/general_io.rb

@@ -7,7 +7,7 @@ class Reline::GeneralIO
   end
 
   def self.encoding
-    if @@encoding
+    if defined?(@@encoding)
       @@encoding
     elsif RUBY_PLATFORM =~ /mswin|mingw/
       Encoding::UTF_8