lib/model: Correctly verify short read blocks (fixes #7333) (#7334)
lib/model: Correctly verify short read blocks (fixes #7333) (#7334)

An untrusted device will receive padded info for small blocks, and hence
sometimes request a larger block than actually exists on disk.
Previously we let this pass because we didn't have a hash to compare to
in that case and we ignored the EOF error based on that.

Now the untrusted device does pass an encrypted hash that we decrypt and
verify. This means we can't check for len(hash)==0 any more, but on the
other hand we do have a valid hash we can apply to the data we actually
read. If it matches then we don't need to worry about the read
supposedly being a bit short.
wweich/syncthing,wweich/syncthing,syncthing/syncthing,syncthing/syncthing,syncthing/syncthing,wweich/syncthing,wweich/syncthing,syncthing/syncthing,syncthing/syncthing,syncthing/syncthing