mgmt: updatehub: Fix possible deref an uninitialized ptr

There are several references to objects[1] at updatehub_probe function.
The structures are decoded from json, and have a maximum length of 2.
However, if the returned json only has a single element in this array,
this objects[1] value will be uninitialized. Because the structure
contains pointers, these will be uninitialized, causing the code to
reference uninitialized memory as pointers.

Add zeroing memory before passing it to the JSON API and do check if
objects_len field is two.

Fixes #27718.

Signed-off-by: Gerson Fernando Budke <gerson.budke@ossystems.com.br>