gst_fd_allocator_alloc() ensure that passed allocator is a fd memory allocator, so that we can't pass NULL allocator. https://bugzilla.gnome.org/show_bug.cgi?id=754833
gst_fd_allocator_alloc() ensure that passed allocator is a fd memory allocator, so that we can't pass NULL allocator. https://bugzilla.gnome.org/show_bug.cgi?id=754833