Fix autovacuum cancellation.

The problem is caused by me (Andres) having ProcSleep() look at the
wrong PGPROC entry in 5788e258bb.

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

src/backend/storage/lmgr/proc.c

@@ -1318,7 +1318,7 @@ ProcSleep(LOCALLOCK *locallock, LockMethod lockMethodTable)
 			 * Only do it if the worker is not working to protect against Xid
 			 * wraparound.
 			 */
-			vacuumFlags = ProcGlobal->vacuumFlags[proc->pgxactoff];
+			vacuumFlags = ProcGlobal->vacuumFlags[autovac->pgxactoff];
 			if ((vacuumFlags & PROC_IS_AUTOVACUUM) &&
 				!(vacuumFlags & PROC_VACUUM_FOR_WRAPAROUND))
 			{