drm/etnaviv: potential NULL dereference

The etnaviv_gem_prime_get_sg_table() is supposed to return error
pointers.  Otherwise it can lead to a NULL dereference when it's called
from drm_gem_map_dma_buf().

Fixes: 5f4a4a73 ("drm/etnaviv: fix gem_prime_get_sg_table to return new SG table")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>

drivers/gpu/drm/etnaviv/etnaviv_gem_prime.c

@@ -15,7 +15,7 @@ struct sg_table *etnaviv_gem_prime_get_sg_table(struct drm_gem_object *obj)
 	int npages = obj->size >> PAGE_SHIFT;
 
 	if (WARN_ON(!etnaviv_obj->pages))  /* should have already pinned! */
-		return NULL;
+		return ERR_PTR(-EINVAL);
 
 	return drm_prime_pages_to_sg(etnaviv_obj->pages, npages);
 }