Fix corner case in autovacuum-forcing logic for multixact wraparound.

 Fix corner case in autovacuum-forcing logic for multixact wraparound.
 
 Since find_multixact_start() relies on SimpleLruDoesPhysicalPageExist(),
 and that function looks only at the on-disk state, it's possible for it
 to fail to find a page that exists in the in-memory SLRU that has not
 been written yet.  If that happens, SetOffsetVacuumLimit() will
 erroneously decide to force emergency autovacuuming immediately.
 
 We should probably fix find_multixact_start() to consider the data
 cached in memory as well as on the on-disk state, but that's no excuse
 for SetOffsetVacuumLimit() to be stupid about the case where it can
 no longer read the value after having previously succeeded in doing so.
 
 Report by Andres Freund.