You have some kind of corruption. You didn't tell us what version you are
using. For me, that file being missing just causes FATAL, not PANIC. I
don't know why it would be different for you.
If you shutdown the database and then start it back up with the -P option
to 'postgres' of "-o -P" for 'pg_ctl', then you should be able to connect
to the database and do a REINDEX SYSTEM. This should fix the problem.
Then you can shut it down and start it up again without the special
option.
However, once you have some kind of corruption there is no telling how much
other corruption you also have. Why would just this one file happen to be
missing?
Cheers,
Jeff