u_vbuf: don't try to delete NULL driver CSO

Since 18a8c3f7f1 we don't create a driver CSO if there are any
incompatible elements, so only ask backends to delete it if it exists.

Fixes multiple CTS crashes in V3D.

Fixes: 18a8c3f7f1 ("u_vbuf: Only create driver CSO if no incompatible elements")

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/gallium/auxiliary/util/u_vbuf.c

@@ -849,7 +849,8 @@ static void u_vbuf_delete_vertex_elements(struct u_vbuf *mgr, void *cso)
    struct pipe_context *pipe = mgr->pipe;
    struct u_vbuf_elements *ve = cso;
 
-   pipe->delete_vertex_elements_state(pipe, ve->driver_cso);
+   if (ve->driver_cso)
+      pipe->delete_vertex_elements_state(pipe, ve->driver_cso);
    FREE(ve);
 }