fixed "Conditional jump or move depends on uninitialised value(s)" in GBD

(cherry picked from commit 887736bc)

modules/ml/src/gbt.cpp

@@ -259,7 +259,7 @@ CvGBTrees::train( const CvMat* _train_data, int _tflag,
         for (int i=1; i<n; ++i)
         {
             int k = 0;
-            while ((int(orig_response->data.fl[i]) - class_labels->data.i[k]) && (k<j))
+            while ((k<j) && (int(orig_response->data.fl[i]) - class_labels->data.i[k]))
                 k++;
             if (k == j)
             {