Fix autovacuum cancellation.

Fix autovacuum cancellation.

The problem is caused by me (Andres) having ProcSleep() look at the
wrong PGPROC entry in 5788e258bb2.

Unfortunately it seems hard to write a reliable test for autovacuum
cancellations. Perhaps somebody will come up with a good approach, but
it seems worth fixing the issue even without a test.

Reported-By: Jeff Janes <jeff.janes@gmail.com>
Author: Jeff Janes <jeff.janes@gmail.com>
Discussion: https://postgr.es/m/CAMkU=1wH2aUy+wDRDz+5RZALdcUnEofV1t9PzXS_gBJO9vZZ0Q@mail.gmail.com