fix SMAC bug that causes multiclass sample to infinite loop (dotnet#209)

Author: daholste

src/Microsoft.ML.Auto/Sweepers/SmacSweeper.cs

@@ -386,6 +386,10 @@ private double ComputeEI(double bestVal, double[] forestStatistics)
             double empMean = forestStatistics[0];
             double empStdDev = forestStatistics[1];
             double centered = empMean - bestVal;
+            if (empStdDev == 0)
+            {
+                return centered;
+            }
             double ztrans = centered / empStdDev;
             return centered * SweeperProbabilityUtils.StdNormalCdf(ztrans) + empStdDev * SweeperProbabilityUtils.StdNormalPdf(ztrans);
         }