deltas: Don't put unreadable *from* objects in fallback

deltas: Don't put unreadable *from* objects in fallback

In https://github.com/ostreedev/ostree/pull/634 we introduced
a subtle regression - the unreadable object was added to the *new*
reachable objects, when it shouldn't have been.  Because it
was a *from* object, clients already had it.

This became more obvious now that I'm working on fixing delta
progress - I noticed my deltas were always starting out with 40MB
fetched, which turned out to be a non-world-readable initramfs object.

This code should simply *skip* the unreadable object, and the delta processing
below properly iterates over "new objects", so we'll pick it up from there.

Closes: #678
Approved by: giuseppe