could not truncate directory "pg_serial": apparent wraparound

We had a report of the subject message during testing a while back
and attempted to address the issue.  It can result in a LOG level
message and the accumulation of files in the pg_serial SLRU
subdirectory.  We haven't seen a recurrence, until I hit it during
testing of the just-posted patch for SSI DDL.  I re-read the code
and believe that the attached is the correct fix.
 
-Kevin