optparse: Suppress warnings

Ruby warns "instance variable `@version` not initialized" for optparse when it receives `--version` option.

`test.rb`

```ruby
require 'optparse'
Version = '1'
OptionParser.new.parse(ARGV)
```

```
$ ruby -w test.rb --version
/home/pocke/.rbenv/versions/2.5.1/lib/ruby/2.5.0/optparse.rb:1168: warning: instance variable @version not initialized
/home/pocke/.rbenv/versions/2.5.1/lib/ruby/2.5.0/optparse.rb:1175: warning: instance variable @release not initialized
test 1
```

This change will suppress the warnings.

[Fix GH-1871]

From: Masataka Pocke Kuwabara <kuwabara@pocke.me>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63342 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/optparse.rb

@@ -1165,14 +1165,14 @@ XXX
   # Version
   #
   def version
-    @version || (defined?(::Version) && ::Version)
+    (defined?(@version) && @version) || (defined?(::Version) && ::Version)
   end
 
   #
   # Release code
   #
   def release
-    @release || (defined?(::Release) && ::Release) || (defined?(::RELEASE) && ::RELEASE)
+    (defined?(@release) && @release) || (defined?(::Release) && ::Release) || (defined?(::RELEASE) && ::RELEASE)
   end
 
   #

test/optparse/test_summary.rb

@@ -48,6 +48,8 @@ class TestOptionParser::SummaryTest < TestOptionParser
   def test_ver
     o = OptionParser.new("foo bar")
     o.program_name = "foo"
+    assert_warning('') {assert_nil(o.version)}
+    assert_warning('') {assert_nil(o.release)}
     o.version = [0, 1]
     assert_equal "foo 0.1", o.ver
     o.release = "rel"