Created Unassigned: Crash when making multiple gifs [19]

I get an IOException right after I press Stop : "The file 'C:\Users\...\AppData\Local\Temp\ScreenToGif\Recording\2014-11-15 03-53-55\Edit\0.bmp' already exists."

```
at System.Windows.Forms.Control.MarshaledInvoke(Control caller, Delegate method, Object[] args, Boolean synchronous)
at System.Windows.Forms.Control.Invoke(Delegate method, Object[] args)
at System.Runtime.Remoting.Messaging.AsyncResult.SyncProcessMessage(IMessage msg)
at System.Runtime.Remoting.Messaging.StackBuilderSink.AsyncProcessMessage(IMessage msg, IMessageSink replySink)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
at System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
at System.Threading.ThreadPoolWorkQueue.Dispatch()
at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback()
```

Steps to reproduce the issue :
* Start recording anything, wait, stop and click on Cancel. On the "Do you want to encode the animation" dialog, use no.
* Start another recording and when you stop this new one, it will throw the exception.