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
lib - sys sys
mercurial - sys sys
hgrc.d - sys sys
9diff.rc - sys sys
factotum.rc - sys sys
rc - sys sys
bin - sys sys
9diff - sys sys
hg - sys sys
sys - sys sys
lib - sys sys
python - sys sys
lib - sys sys
python2.5 - sys sys
site-packages - sys sys
hgext - sys sys
+ - sys sys
mercurial - sys sys
+ - sys sys
mercurial-VERSION-py2.5.egg-info - sys sys
src - sys sys
cmd - sys sys
hg - sys sys
+ - sys sys