TST check that single-leaf trees don't cause segfaults (#852)
TST check that single-leaf trees don't cause segfaults (#852)


wxchan/LightGBM,henry0312/LightGBM,Allardvm/LightGBM,wxchan/LightGBM,Allardvm/LightGBM,microsoft/LightGBM,Allardvm/LightGBM,wxchan/LightGBM,henry0312/LightGBM,olofer/LightGBM,microsoft/LightGBM,Allardvm/LightGBM,Laurae2/LightGBM,microsoft/LightGBM,olofer/LightGBM,olofer/LightGBM,microsoft/LightGBM,Allardvm/LightGBM,Laurae2/LightGBM,henry0312/LightGBM,Laurae2/LightGBM,olofer/LightGBM,Laurae2/LightGBM,wxchan/LightGBM,henry0312/LightGBM,microsoft/LightGBM,olofer/LightGBM,wxchan/LightGBM,Laurae2/LightGBM,henry0312/LightGBM