chromaprint: fix dereferencing dangling pointer

I realized in the Windows nightly build that in the Qt interface closing the dialog before fingerprinting is complete causes crash because of this.

I'm not sure if this is the best way to fix it, feel free to come up with a better fix. I can then close this merge request.

I previously tried to handle closing the dialog before completion in !5100 (merged), but I did not realize this crash before (chromaprint was not installed on my development environment).