Avoid memcpy() with same source and destination in pgstat_recv_replslot.

Same type of issue as in commit 53d4f5fef and earlier fixes; also
found by apparently-more-picky-than-the-buildfarm valgrind testing.
This one is an oversight in commit 986816750.  Since that's new in
HEAD, no need for a back-patch.

src/backend/postmaster/pgstat.c

@@ -6916,15 +6916,15 @@ pgstat_recv_replslot(PgStat_MsgReplSlot *msg, int len)
 		return;
 
 	/* it must be a valid replication slot index */
-	Assert(idx >= 0 && idx < max_replication_slots);
+	Assert(idx < nReplSlotStats);
 
 	if (msg->m_drop)
 	{
 		/* Remove the replication slot statistics with the given name */
+		if (idx < nReplSlotStats - 1)
 			memcpy(&replSlotStats[idx], &replSlotStats[nReplSlotStats - 1],
 				   sizeof(PgStat_ReplSlotStats));
 		nReplSlotStats--;
-		Assert(nReplSlotStats >= 0);
 	}
 	else
 	{