[gstreamer] unref the buffer when we clear the buffer pool. Fixes JB#11011
[gstreamer] unref the buffer when we clear the buffer pool. Fixes JB#11011

We forgot to unref the buffer and were leaking it.
The buffer should be unreffed when we remove it from the buffers queue. We
are sure that it's neither in app queue nor in hal queue so we can safely
destroy it.

sailfishos/gst-droidcamsrc,sailfishos/gst-droidcamsrc,sailfishos/gst-droidcamsrc