fix compiler uninitialized variable

fix compiler uninitialized variable

In file included from ubus.c:20:
ubus.c: In function 'uh_ubus_list_cb':
libubox/blobmsg.h:256:9: error: 'o' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  256 |         blob_nest_end(buf, cookie);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~
ubus.c:591:19: note: 'o' was declared here
  591 |         void *t, *o;
      |                   ^
cc1: all warnings being treated as errors

Signed-off-by: Thomas Huehn <thomas.huehn@hs-nordhausen.de>
Acked-by: Alexander Couzens <lynxis@fe80.eu>