Fix memory leak in Image#recolor (#454)
Fix memory leak in Image#recolor (#454)

`NUM2DBL()` will raise exception if non-float value is given.

The allocated memory area using `ALLOC_N()` and `AcquireExceptionInfo()` cause memory leak.

* Before

```
$ ruby test.rb
Process: 27355: RSS = 185 MB
```

* After

```
$ ruby test.rb
Process: 30252: RSS = 14 MB
```

* Test code

```ruby
require 'rmagick'

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

400000.times do |i|
  begin
    image.recolor(['x', 1, 1, 1])
  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