etnaviv: Allow clearing constant buffer using buffer==NULL user_buffer==NULL

Prevents an assertion when using GALLIUM_HUD with ioquake3,
when cso_restore_constant_buffer_slot0 restores an empty
constant buffer in slot 0.

Signed-off-by: Wladimir J. van der Laan <laanwj@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>

src/gallium/drivers/etnaviv/etnaviv_state.c

@@ -89,7 +89,7 @@ etna_set_constant_buffer(struct pipe_context *pctx,
 
    /* Note that the state tracker can unbind constant buffers by
     * passing NULL here. */
-   if (unlikely(!cb))
+   if (unlikely(!cb || (!cb->buffer && !cb->user_buffer)))
       return;
 
    /* there is no support for ARB_uniform_buffer_object */