gst: fix NULL pointer when listing devices

The gstdeviceprovider segfaults when listing the available devices,
because it the pointer to GList * is initialized with NULL instead of
the GList * itself. Don't use a pointer, but use the GList * directly.

src/gst/gstpipewiredeviceprovider.h

@@ -96,7 +96,7 @@ struct _GstPipeWireDeviceProvider {
   int error;
   gboolean end;
   gboolean list_only;
-  GList **devices;
+  GList *devices;
 };
 
 struct _GstPipeWireDeviceProviderClass {