assert about the control message before it's about to be used, not after it's just been advanced.  it's valid to return NULL after the last iteration.
assert about the control message before it's about to be used, not after it's just been advanced.  it's valid to return NULL after the last iteration.

git-svn-id: 81e381228600e5752b80483efd2b45b26c451ea2@7850 e27351fd-9f3e-4f54-a53b-843176b1656c

trevor/calendarserver,trevor/calendarserver,trevor/calendarserver