When deleting star refs on a change deletion the star refs are computed
from the index which may be stale. If the ref that we want to delete
doesn't exist that's fine and we can just skip it.
This NPE was observed when a change deletion failed with LOCK_FAILURE
and then the request was automatically retried.
Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: Icb73fdfd5bf6141176b733a25809c59e7f4d5c22
c3769b771b