Tree iterators return garbage if an index is modified between calls

Tree iterator is not invalidated by deletion from an index:

localhost> lua box.space[16]:insert('a', 'a', 'a')
---
 - 'a': {'a', 'a'}
...
localhost> lua box.space[16]:insert('b', 'b', 'b')
---
 - 'b': {'b', 'b'}
...
localhost> lua box.space[16]:insert('c', 'c', 'c')
---
 - 'c': {'c', 'c'}
...
localhost> lua box.space[16]:insert('d', 'd', 'd')
---
 - 'd': {'d', 'd'}
...
localhost> lua box.space[16]:insert('e', 'e', 'e')
---
 - 'e': {'e', 'e'}
...
localhost> lua k,v = box.space[16].index[1]:next(k)
---
...
localhost> lua print(k, " ", v)
---
userdata: 0x41544568 'a': {'a', 'a'}
...
localhost> lua k,v = box.space[16].index[1]:next(k)
---
...
localhost> lua print(k, " ", v)
---
userdata: 0x41544568 'b': {'b', 'b'}
...
localhost> lua box.space[16]:truncate()
---
...
localhost> lua print(k, " ", v)
---
userdata: 0x41544568 'b': {'b', 'b'}
...
localhost> lua collectgarbage('collect')
---
 - 0
...
localhost> lua print(k, " ", v)
---
userdata: 0x41544568 'b': {'b', 'b'}
...
localhost> lua k,v = box.space[16].index[1]:next(k)
---
...
localhost> lua print(k, " ", v)
---
userdata: 0x41544568 'c': {'c', 'c'}
...
localhost> lua collectgarbage('collect')
---
 - 0
...
localhost> lua print(k, " ", v)
---
userdata: 0x41544568 'c': {'c', 'c'}
...
localhost> lua k,v = box.space[16].index[1]:next(k)
---
...
localhost> lua print(k, " ", v)
---
userdata: 0x41544568 'd': {'d', 'd'}
...
localhost> lua k,v = box.space[16].index[1]:next(k)
---
...
localhost> lua print(k, " ", v)
---
userdata: 0x41544568 'e': {'e', 'e'}
...
localhost> lua k,v = box.space[16].index[1]:next(k)
---
...
localhost> lua print(k, " ", v)
---
nil nil
...
localhost> quit