tags: add safety check for len(record) while reading hgtagsfnodescache I am trying to fix a breakage where somehow we end up getting a node of 12 length from `getfnode()`. Understanding the hgtagsfnodescache code, it seems highly unlikely that it can happen unless one of `mctx.readfast().get()` or `ctx.filenode()` is returning a node of 12 length. For safety, I think it's better to add a check to make sure that record which we are parsing is of same length we are expecting otherwise we consider that as invalid record. Differential Revision: https://phab.mercurial-scm.org/D9169

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