I’m not sure exactly how this code is supposed to work, so this might
not be the right fix. But there’s definitely a problem here, and it was
spotted by scan-build.

If `param_value_array_validate()` is entered with
`value->data[0].v_pointer == NULL && aspec->fixed_n_elements`, that `NULL`
will be stored in `value_array` too. `value->data[0].v_pointer` will
then be set to a new non-`NULL` array.

A few lines down, `value_array_ensure_size()` is called on
`value_array` – which is still `NULL` – and this results in a `NULL`
pointer dereference.

It looks like `value->data[0].v_pointer` and `value_array` are used
interchangeably throughout the whole of the function, so assign the new
value of `value->data[0].v_pointer` to `value_array` too.

My guess is that `value_array` is just a convenience alias for
`value->data[0].v_pointer`, because the latter is a real mouthful to
type or read.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #1767