Underflow in MerkleTree

[ed255]

When the tree is empty new_siblings.len() is 0. This triggers an underflow in the code:

pod2/src/backends/plonky2/primitives/merkletree/circuit.rs

Line 649 in c382bf4

F::from_canonical_u64((new_siblings.len() - 1) as u64),

The self.divergence_level is not used in this situation so the value assigned there doesn't matter, but I think we should fix the underflow condition, which is checked by rust in debug mode and panics.

A solution could be to do a conditional assignment of 0 if new_siblings.len() == 0