Use XLogRecPtrIsValid() instead of negated XLogRecPtrIsInvalid

Hi,

This small cleanup patch updates src/bin/pg_waldump/archive_waldump.c
to use the recently introduced XLogRecPtrIsValid() helper instead of
negating XLogRecPtrIsInvalid(). The current code uses double-negative
checks such as:
Assert(!XLogRecPtrIsInvalid(privateInfo->startptr));
if (!XLogRecPtrIsInvalid(privateInfo->endptr))

This patch changes them to:
Assert(XLogRecPtrIsValid(privateInfo->startptr));
if (XLogRecPtrIsValid(privateInfo->endptr))

This improves readability without changing behavior. The attached
patch has the changes for the same.

Regards,
Vignesh