[staticanalysis]: Actually check errors on splice() of objects

[staticanalysis]: Actually check errors on splice() of objects

We were using unsigned size when we should have been using signed,
this means we basically weren't checking for errors on write...ouch.

Luckily if we e.g. hit ENOSPC during a pull, the checksums wouldn't
match and we'd return an error anyways.  However when writing an
object, we'd end up silently ignoring it =/

https://bugzilla.gnome.org/show_bug.cgi?id=732020