fix invalid close() call

fix invalid close() call

valgrind complained about this one

==18632== Warning: invalid file descriptor -1 in syscall close()
==18632==    at 0x5326D20: __close_nocancel (syscall-template.S:84)
==18632==    by 0x5046C02: ubus_process_invoke (libubus-obj.c:98)
==18632==    by 0x5046DC3: ubus_process_obj_msg (libubus-obj.c:142)
==18632==    by 0x5045E98: ubus_process_msg (libubus.c:106)
==18632==    by 0x50468D0: ubus_handle_data (libubus-io.c:314)
==18632==    by 0x4E3D125: uloop_run_events (uloop.c:198)
==18632==    by 0x4E3D125: uloop_run_timeout (uloop.c:555)
==18632==    by 0x109BEF: uloop_run (uloop.h:111)
==18632==    by 0x109BEF: main (main.c:25)

Signed-off-by: John Crispin <john@phrozen.org>