authgraph: cope with LocalTa being dropped

When a LocalTa instance is used in a fuzzer, it gets dropped at the end
of processing each fuzz input.  This makes the `mpsc::channel`s fail,
so update to cope with this.

Bug: 316075932
Test: run fuzzer on Cuttlefish
Change-Id: I069f441013e269f652cbe1ff3053606f9bcb2dfd