Correct Pruning for k >= 2

[kanodiaayush]

https://github.com/grf-labs/policytree/blob/master/src/tree_search.cpp#L334

At this point, pruning should be incorrect since we treat the non pruned optimal nodes differently from the pruned, and in the case that the pruned node is more optimal than two non pruned nodes it splits into, we will report the non pruned always.