atspi_accessible_dispose: Avoid unreferencing NULL
atspi_accessible_dispose: Avoid unreferencing NULL

The children array may contain NULL entries, when the children have not
all been inspected yet for instance.  In such a case
atspi_accessible_dispose's call to g_ptr_array_free would warn out when
freeing array items with g_object_unref.  We should thus check for NULL
to avoid such warning.

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