unroot error in optim.pml

[qifei9]

When an alignment has only 2 unique sequences after dropping duplicated tips, the optim.pml with rearrangement = 'stochastic', 'NNI' or 'ratchet' will run into error like: cannot unroot the tree. Perhaps due to line 2468 in R/phylo.R and maybe a check should be added before unroot.