util: avoid a name-error warning in the `mmapread` exception handler
Newly caught by pytype 2021-09-09.
File "/mnt/c/Users/Matt/hg/mercurial/util.py", line 458, in mmapread: Name 'fd' is not defined [name-error]
Differential Revision: https://phab.mercurial-scm.org/D11470
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