drm/vmwgfx: Make vmw_dmabuf_unreference handle NULL objects

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Dmitry Torokhov <dtor@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/vmwgfx/vmwgfx_drv.h

@@ -699,10 +699,13 @@ static inline struct vmw_surface *vmw_surface_reference(struct vmw_surface *srf)
 static inline void vmw_dmabuf_unreference(struct vmw_dma_buffer **buf)
 {
 	struct vmw_dma_buffer *tmp_buf = *buf;
-	struct ttm_buffer_object *bo = &tmp_buf->base;
+
 	*buf = NULL;
+	if (tmp_buf != NULL) {
+		struct ttm_buffer_object *bo = &tmp_buf->base;
 
 		ttm_bo_unref(&bo);
+	}
 }
 
 static inline struct vmw_dma_buffer *vmw_dmabuf_reference(struct vmw_dma_buffer *buf)