Iterator disobeys maxKey after doc deletion

During Couchbase Lite testing today we found a regression where the ToDoLite app displays incorrect items in a list after deleting an item. (https://github.com/couchbase/couchbase-lite-ios/issues/1370). I've tracked the cause to a bug in fdb_iterator, where it returns a document that's outside the requested key range.

To reproduce:
1. Check out couchbase-lite-ios, branch "fix/iterator_fail_1370"
2. Run unit test -[ViewTests test25_ToDoLiteTaskListQuery]
3. Test will log warnings and fail.

In ForestDB terms, the steps to reproduce are something like:
1. Create documents with keys A, B
2. Create an iterator with minKey=B, maxKey=Bzz
3. Call fdb_iterator_seek_to_max
4. Call fdb_iterator_get
--> Correctly returns doc B
5. Delete doc B
6. Repeat steps 2-4
--> Incorrectly returns doc A; I expected fdb_iterator_get to return FDB_RESULT_ITERATOR_FAIL since there is nothing to enumerate

(I haven't verified that a test this simple using only ForestDB APIs would reproduce the issue. The Couchbase Lite test uses higher level APIs but should result in ForestDB calls similar to these.)