dri2: check if context is valid before flushing the pipe

src/gallium/state_trackers/dri/drm/dri2.c

@@ -49,7 +49,8 @@ dri2_flush_drawable(__DRIdrawable *draw)
    struct dri_drawable *drawable = dri_drawable(draw);
    struct dri_context *ctx = dri_get_current(draw->driScreenPriv);
 
-   ctx->st->flush(ctx->st, 0, NULL);
+   if (ctx)
+      ctx->st->flush(ctx->st, 0, NULL);
 }
 
 static void