Fix memory leak in Image._load (#391)
Fix memory leak in Image._load (#391)

If If invalid argument was given, `rm_str2cstr()` will raise exception.
Then, the memory area allocated by `rm_clone_image()` causes memory leak.

* Before

```
$ ruby test.rb
Process: 39019: RSS = 176 MB
```

* After

```
$ ruby test.rb
Process: 40111: RSS = 14 MB
```

* Test code

```ruby
require 'rmagick'

10000.times do |i|
  begin
    Magick::Image._load(Object.new)
  rescue
  end

  GC.start if i % 100 == 0
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