virJSONValueArrayAppend: Clear pointer when taking ownership of passed value

virJSONValueArrayAppend: Clear pointer when taking ownership of passed value

The parent array takes ownership of the inserted value once all checks
pass. Don't make the callers second-guess when that happens and modify
the function to take a double pointer so that it can be cleared once the
ownership is taken.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>