drm/radeon: avoid potential null pointer access

Leverage the same logic from amdgpu_ttm_tt_unpin_userptr.
Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_ttm.c

@@ -415,7 +415,7 @@ static void radeon_ttm_tt_unpin_userptr(struct ttm_device *bdev, struct ttm_tt *
 		DMA_BIDIRECTIONAL : DMA_TO_DEVICE;
 
 	/* double check that we don't free the table twice */
-	if (!ttm->sg->sgl)
+	if (!ttm->sg || !ttm->sg->sgl)
 		return;
 
 	/* free the sg table and pages again */