fix immutable hash tree balancing
fix immutable hash tree balancing

When an existing entry in the AVL tree for an immutable hash
is replaced with a new one, the new one did not always get
the right depth, which could throw off the tree balance.

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket