Apparently it may happen that some fly does get a different anchor node
than before, which means the RemoveAnchoredFly() will be called twice.

Prevent the problem differently by clearing the anchor in
_SaveFlyInRange().

(regression from a624cf52)

Change-Id: Id796d2e230de065113bcf1e6164d6b8a01ab933c
(cherry picked from commit 8a1f5536)