hasher: fix crash on object not found

Before this fix `NewObject` could return a wrapped `fs.Object(nil)`
which caused a crash.

This changes the interface of `wrapObject` so it returns an error
which must be checked. This forces the callers to return a `nil`
object rather than an `fs.Object(nil)`.

See: https://forum.rclone.org/t/panic-in-hasher-when-mounting-with-vfs-cache-and-not-synced-data-in-the-cache/29697/11

backend/hasher/object.go

@@ -210,8 +210,8 @@ func (f *Fs) Put(ctx context.Context, in io.Reader, src fs.ObjectInfo, options .
 
 	_ = f.pruneHash(src.Remote())
 	oResult, err := f.Fs.Put(ctx, wrapIn, src, options...)
-	o = f.wrapObject(oResult, err)
-	if o == nil {
+	o, err = f.wrapObject(oResult, err)
+	if err != nil {
 		return nil, err
 	}