Fix memory leak in Image#convolve (#450)
Fix memory leak in Image#convolve (#450)

`NUM2DBL()` will raise exception if non-float value is given.
The allocated memory area using `ALLOC_N()` should be freed before raising.

* Before

```
$ ruby test.rb
Process: 14895: RSS = 53 MB
```

* After

```
$ ruby test.rb
Process: 16886: RSS = 14 MB
```

* Test code

```ruby
require 'rmagick'

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

400000.times do |i|
  begin
    image.convolve(3, [1.0, 1.0, 1.0, 1.0, 'x', 1.0, 1.0, 1.0, 1.0])
  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