Bug 2056116 - Don't crash sync on an invalid login guid

[jo]

Serializing an outgoing record whose guid fails our sync-server sanity check asserted, which took down the parent process. It's now a serde error, and the logins engine skips such a record and reports it as logins-invalid-outgoing-guid, so a single login can't block the whole sync. Incoming records were already handled this way.

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