Fixed a bug which triggered when using variable length matrices

as the feature vector type.

dlib/svm/structural_svm_assignment_problem.h

@@ -94,6 +94,7 @@ namespace dlib
         ) const 
         {
             typename feature_extractor::feature_vector_type feats;
+            psi.set_size(fe.num_features());
             psi = 0;
             for (unsigned long i = 0; i < sample.first.size(); ++i)
             {