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

mercurial/tags.py

@@ -759,7 +759,7 b' class hgtagsfnodescache(object):'
         properprefix = node[0:4]
 
         # Validate and return existing entry.
-        if record != _fnodesmissingrec:
+        if record != _fnodesmissingrec and len(record) == _fnodesrecsize:
             fileprefix = record[0:4]
 
             if fileprefix == properprefix: