Fix pg_waldump to exit cleanly at end of WAL

Hi Hackers,

I noticed a problem with pg_waldump. When "-e" is not specified, or given a
bigger number beyond the current LSN, pg_dump always print an error message
like:

```
chaol(at)ChaodeMacBook-Air cndb % pg_waldump -p pg_wal -s 0/2980DE10 -e
0/2980E680
<...omit some outputs...>
rmgr: Standby len (rec/tot): 50/ 50, tx: 0, lsn:
0/2980E468, prev 0/2980E3F0, desc: RUNNING_XACTS nextXid 866
latestCompletedXid 865 oldestRunningXid 866
pg_waldump: error: error in WAL record at 0/2980E468: invalid record length
at 0/2980E4A0: expected at least 24, got 0
```

The error message is kinda misleading and unnecessary. So I created a fix.

From the code comment in pg_waldump.c, the same change should be applied to
pg_walinspect. If this patch is accepted, I will then work on pg_walinspect
as well.

Best regards,
Chao Li (Evan)
---------------------
HighGo Software Co., Ltd.
https://www.highgo.com/