json_pointer: move array out-of-bounds check outside of is_valid_index()

json_pointer: move array out-of-bounds check outside of is_valid_index()

The out-of-bounds check is useful when trying to index/obtain a value from
an array.
However, when we set a value to a specific JSON pointer, we can allow
values that are outside the length of the current array.
The RFC6901 doc isn't clear on that aspect, and doing so is a bit more
in-line with how json_object_array_{put,insert}_idx() functions behave.

This changes the behavior of json_pointer_set{f}() because now a value can
be set anywhere in the array.

Also, added a test-case for this behavior change.

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