wayland_shm: Fix extra buffer allocation on client shutdown

On shutdown we need to set wl_surface to NULL to let the engine know
there's nothing to write to - however we've been setting up an entirely
new outbuf instead of just changing the old one.

src/modules/evas/engines/wayland_shm/evas_engine.c

@@ -157,7 +157,7 @@ eng_setup(Evas *eo_evas, void *info)
         else
           goto err;
      }
-   else
+   else if (einfo->info.wl_surface)
      {
         Outbuf *ob;