Spotted by Mohammed Sadiq. `g_array_copy()` was doing a `memcpy()` of
the data from the old array to the new one, based on the reserved
elements in the old array (`array->alloc`). However, the new array was
allocated based on the *assigned* elements in the old array
(`array->len`).

So if the old array had fewer assigned elements than allocated elements,
`memcpy()` would fall off the end of the newly allocated data block.
This was particularly obvious when the old array had no assigned
elements, as the new array’s data pointer would be `NULL`.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Fixes: #2049