podcheck.t: Guard against transitory files

podcheck.t: Guard against transitory files

podcheck uses File::Find to look for pods in the directory structure.
It is possible for the Find to find a transitory file that is gone by
the time the file is opened for examination.  This patch will simply
skip the transitory file if the open fails and it's because it no longer
exists.  (There is a tiny window in which the open could fail, and
the file gets recreated and hence passes the exists test, but I'm not
bothering to code for that until it actually happens.)