Fix memory leak in Image#shear (#382)
Fix memory leak in Image#shear (#382)

If invalid argument was given, `NUM2DBL()` will raise an exception.
Then, the memory area allocated by `AcquireExceptionInfo()` causes memory leak.

* Before

```
$ ruby test.rb
Process: 15476: RSS = 88 MB
```

* After

```
$ ruby test.rb
Process: 16770: RSS = 14 MB
```

* Test code

```ruby
require 'rmagick'

image = Magick::Image.new(20, 20)

200000.times do |i|
  begin
    image.shear('x', 'y')
  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