mail-client: Simplify the exit paths
mail-client: Simplify the exit paths

Now that the GTask is passed around instead of the CheckData, we can
simplify the exit paths by coupling the ref / unref pairs more
explicitly. This was not possible earlier because CheckData is not a
ref-counted type.

https://bugzilla.gnome.org/show_bug.cgi?id=764157

ritzk/gnome-online-accounts,ritzk/gnome-online-accounts,ritzk/gnome-online-accounts