Per the conventions around using GError, failure code paths must set a
GError if a non-NULL pointer is passed for that parameter. And in case
the error is left unset it leads to an assertion failure (and crash) in
flatpak_installation_list_installed_refs_for_update(); see
https://github.com/endlessm/flatpak/pull/224
So add assertions to the failure code paths in
flatpak_transaction_real_run() to catch such mistakes earlier and make
the resulting crash stack trace more useful, since it points closer to
where the problem is. This could be implemented with the "goto out;"
idiom but I don't think that's a good idea in this case because in the
latter half of the function we have to do some cleanup before returning.
a167252492