vout: fix use-after-free of SPU

Pointed-out-by: Pedro Ribeiro <pedrib@gmail.com>

src/video_output/video_output.c

@@ -1034,8 +1034,10 @@ static int ThreadDisplayRenderPicture(vout_thread_t *vout, bool is_forced)
                 picture_BlendSubpicture(sys->display.filtered, vout->p->spu_blend, subpic);
             vout_display_Prepare(vd, sys->display.filtered, do_dr_spu ? subpic : NULL);
         }
-        if (!do_dr_spu && subpic)
+        if (!do_dr_spu && subpic) {
             subpicture_Delete(subpic);
+            subpic = NULL;
+        }
         if (!sys->display.filtered)
             return VLC_EGENERIC;
     }