Fix memory leak in Info#format= (#361)
Fix memory leak in Info#format= (#361)

If the object which can't be converted to `String`, Ruby C API `StringValuePtr()` will raise the exception.
Then, memory area allocated by `AcquireExceptionInfo()` causes memory leak.

* Before

```
$ ruby format.rb
Process: 32550: RSS = 398 MB
```

* After

```
$ ruby format.rb
Process: 34253: RSS = 16 MB
```

* Test code

```ruby
require 'rmagick'

info = Magick::Image::Info.new

1000000.times do
  begin
    info.format = Object.new
  rescue
  end
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