atspi_accessible_get_parent: move check for NULL AtspiApplication object
atspi_accessible_get_parent: move check for NULL AtspiApplication object

Now, if we don't have a cached parent, then we always either return NULL
or make a D-Bus call. This might make the code more robust, and hoping
that it will fix https://bugzilla.gnome.org/show_bug.cgi?id=767074,
though in theory it should make no difference there.

GNOME/at-spi2-core,GNOME/at-spi2-core,GNOME/at-spi2-core