garray: Fix NULL-termination of GPtrArray copies
The code was accidentally NULL-terminating the source array rather than the copy.
This fixes commit ee247c0a.
Spotted by the array-test
installed test in
https://gitlab.gnome.org/GNOME/glib/-/jobs/2047993.
Signed-off-by: Philip Withnall pwithnall@endlessos.org