Fix snapshot handling bug in recent BRIN fix

Commit a95e3d84c0 added ActiveSnapshot push+pop when processing
work-items (BRIN autosummarization), but forgot to handle the case of
a transaction failing during the run, which drops the snapshot untimely.
Fix by making the pop conditional on an element being actually there.

Author: Álvaro Herrera <alvherre@kurilemu.de>
Backpatch-through: 13
Discussion: https://postgr.es/m/202511041648.nofajnuddmwk@alvherre.pgsql

src/backend/postmaster/autovacuum.c

@@ -2682,6 +2682,7 @@ deleted:
 
 		PushActiveSnapshot(GetTransactionSnapshot());
 		perform_work_item(workitem);
+		if (ActiveSnapshotSet())	/* transaction could have aborted */
 			PopActiveSnapshot();
 
 		/*