libubus: add null for ubus_context object in ubus_shutdown()

At the moment, we do our own null checks before calling ubus_free().
The likely-hood of 'ctx' being null (in ubus_free()) is low,
but since free() handles null, might make sense for ubus_free() to do so as well.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

libubus.c

@@ -358,6 +358,8 @@ struct ubus_context *ubus_connect(const char *path)
 void ubus_shutdown(struct ubus_context *ctx)
 {
 	blob_buf_free(&b);
+	if (!ctx)
+		return;
 	close(ctx->sock.fd);
 	free(ctx->msgbuf.data);
 }