Fix erroneous memory context switch in autovacuum, which was returning to a

Fix erroneous memory context switch in autovacuum, which was returning to a
context long after it had been destroyed.

Per problem report from Justin Pasher.  Patch by Tom Lane and me.

8.3 and later do not have this bug, because this code has been restructured for
unrelated reasons.  In 8.2 it does not manifest as a crash, but it still seems
safer fixing it nonetheless.