[utils] fixed helper function, to avoid dividing by zero

bob/pad/base/utils/helper_functions.py

@@ -276,7 +276,10 @@ def mean_std_normalize(features,
 
     for row in features:  # row is a sample
 
-        row_norm = (row - features_mean) / features_std
+        if features_std.all() != 0:
+          row_norm = (row - features_mean) / features_std
+        else:
+          row_norm = (row - features_mean) 
 
         row_norm_list.append(row_norm)