ostree_raw_file_to_content_stream: Make size default to 0

ostree_raw_file_to_content_stream: Make size default to 0

Some existing code calls into ostree_raw_file_to_content_stream() with
file objects that do not have the standard::size attribute. Since GLib
2.76.0, attempting to access the size of such an object raises a
critical warning. Handle this more gracefully by defaulting the size
to 0, like earlier versions of GLib did.

Signed-off-by: Simon McVittie <smcv@collabora.com>