Fix memory leak in Info#origin= (#348)
Fix memory leak in Info#origin= (#348)

`GetPageGeometry()` API returns allocated memory area.
We need to release it after used or before raising exception.

* Before

```
$ ruby test.rb
Process: 42227: RSS = 1052 MB
```

* After

```
$ ruby test.rb
Process: 42536: RSS = 65 MB
```

* Test code

```ruby
require 'rmagick'

100000.times do
  info = Magick::Image::Info.new
  begin
    # origin= will raise exception.
    info.origin = 'foobarbaz' * 100
  rescue
  end

  info.origin = '+50+20'
end

GC.start
rss = `ps -o rss= -p #{Process.pid}`.to_i / 1024
puts "Process: #{Process.pid}: RSS = #{rss} MB"
rmagick/rmagick,rmagick/rmagick,rmagick/rmagick