Don't attempt to copy to ip_changes in PageArchive class.

This actually happens in Revision::insertOn, so the code is redundant,
but also shouldn't ever execute because it erroneously was checking
for ar_rev_id=0 and not ar_user=0.

This is sometimes causing errors in production, in the rare cases
where ar_rev_id is NULL, and hence (int)ar_rev_id evaluates to 0.

Change-Id: I35a7310f600a9db683d7dc0e730da8ff31e337a2