- When the search in AVL tree went to the leftmost node, it did
  not return any 'previous' node. This was fixed in the AVL
  implementation by distinguishing this case. It may have impact
  on the performance as there is now one more condition jump
  for each encountered node during search. But it seems there is
  no better way around it.
- Fixed the usages of the tree-finding functions.

fixes #1065 @2h