Restoring non-binary trees from state file results in null children

[bjoelle]

Tested with the following tree ((4:0.150558013,5:0.150558013)7:1.154521107,(((2:0.2610234442,3:0.2610234442)9:0.02397655557)10:0.006852994,1:0.2918529938)8:1.013226126)6;

BEAST2 runs fine with it but restoring state fails with a NullPointerException As far as I can tell the issue comes from Tree.assignFrom, which will assign a right null child to node sink if node src has only one child