gdatainputstream: Annotate (out) length arguments as (optional)
gdatainputstream: Annotate (out) length arguments as (optional)

All of them are.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=783350

johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,endlessm/glib,johne53/MB3Glib