GInitable, GAsyncInitable: not initializing gives undefined behaviour

This is the ISO C sense of undefined behaviour, in which
works-by-coincidence, critical warning, abort, demons-fly-out-of-your-nose
are all valid implementations.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=662208


Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: David Zeuthen <davidz@redhat.com>