[HACKERS] Fix comment in xlog.c

Attached a patch for $SUBJECT.

- * If RecPtr is not NULL, try to read a record at that position.  Otherwise
+ * If RecPtr is valid, try to read a record at that position.  Otherwise

Commit 4d6d425ab8d addressed the comment above XLogReadRecord() in
xlogreader.c, but missed the same above ReadRecord() in xlog.c.

Backpatchable to 9.3, I'd think.

Thanks,
Amit

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