Fix uninitialized variable in cubeb_enumerate_devices

[chaoticgd]

I was getting some crashes in cubeb_enumerate_devices using the ALSA backend. I looked into it and found that snd_pcm_open was returning an error code, which caused alsa_enumerate_devices to return before initializing the collection out parameter. This caused the logging loop at the bottom of cubeb_enumerate_devices to read uninitialized data for collection->count.

The documentation doesn't seem to mention anything about having to initialize the collection parameter before calling cubeb_enumerate_devices, and your test code doesn't do so either, so I concluded that this was a bug in Cubeb.

I decided to fix this by zeroing out the collection object because that makes it somewhat consistent with the function below.

Now to figure out how to fix my ALSA...