[droideglsink] Don't leak the acquired buffer. Contributes to JB#17654
[droideglsink] Don't leak the acquired buffer. Contributes to JB#17654

Initialize the buffer pointers in the init function not when the
sink is started, as there's no guarantee the thread that acquired
the buffer has had an opportunity to release it in the time it has
taken another thread to start and stop the sink.

sailfishos/gst-jolla,sailfishos/gst-jolla