drm/radeon: check bo_va->bo is non-NULL before using it

The call to radeon_vm_clear_freed might clear bo_va->bo, so
we have to check it before dereferencing it.
Signed-off-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_gem.c

@@ -642,7 +642,7 @@ static void radeon_gem_va_update_vm(struct radeon_device *rdev,
 	if (r)
 		goto error_unlock;
 
-	if (bo_va->it.start)
+	if (bo_va->it.start && bo_va->bo)
 		r = radeon_vm_bo_update(rdev, bo_va, bo_va->bo->tbo.resource);
 
 error_unlock: