Change inv_truncate() to not repeat its systable_getnext_ordered() scan.

In the case where the initial call of systable_getnext_ordered() returned
NULL, this function would nonetheless call it again.  That's undefined
behavior that only by chance failed to not give visibly incorrect results.
Put an if-test around the final loop to prevent that, and in passing
improve some comments.  No back-patch since there's no actual failure.

Per report from YAMAMOTO Takashi.