providerfactory: Add 'error' precondition
providerfactory: Add 'error' precondition

Ensure that the 'error' parameter is either NULL, or pointing to a NULL
value.

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

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