Fix: replicate gst_sample_get_buffer{,list} returns NULL semantic

[atishnazir]

gst_sample_get_buffer{,list} returns NULL if a buffer{,list} has not been set. Previous go-gst behaviour wrapped this NULL pointer in the respective go-gst proxy object.

This causes a problem with AppSink when one needs to disambiguate between whether a GstSample contains a GstBufferList, a GstBuffer or indeed nothing at all. For example:

sink := app.SinkFromElement(element)
sink.SetBufferListSupport(true)
sink.SetCallbacks(&app.SinkCallbacks{
	NewSampleFunc: func(sink *app.Sink) gst.FlowReturn {
		if sample := sink.PullSample(); sample == nil {
			return gst.FlowEOS
		} else if bufferList := sample.GetBufferList(); bufferList != nil {
			// consume bufferlist
			return gst.FlowOK
		} else if buffer := sample.GetBuffer(); buffer != nil {
			// consume buffer
			return gst.FlowOK
		} else {
			return gst.FlowError
		}
	},
})

[atishnazir]

I debated whether a better fix would be to be to adjust the semantic of FromGstBufferUnsafeNone and FromGstBufferListUnsafeNone to return nil when called with C.NULL but felt the impact would be more far reaching.

[RSWilli]

@atishnazir please recreate this PR for https://github.com/go-gst/go-gst (where future development of the bindings will take place) if you think it is necessary.