Added a missing requirement to sort_basis_vectors()

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403895

dlib/svm/sort_basis_vectors.h

@@ -63,11 +63,13 @@ namespace dlib
         )
         {
             DLIB_ASSERT(is_binary_classification_problem(samples, labels) &&
-                        0 < eps && eps <= 1,
+                        0 < eps && eps <= 1 && 
+                        basis.size() > 0,
                         "\t void sort_basis_vectors()"
                         << "\n\t Invalid arguments were given to this function."
                         << "\n\t is_binary_classification_problem(samples, labels): " << is_binary_classification_problem(samples, labels)
-                        << "\n\t eps: " << eps 
+                        << "\n\t basis.size(): " << basis.size() 
+                        << "\n\t eps:          " << eps 
             );
 
             typedef typename kernel_type::sample_type sample_type;

dlib/svm/sort_basis_vectors_abstract.h

@@ -30,6 +30,7 @@ namespace dlib
         requires
             - is_binary_classification_problem(samples, labels)
             - 0 < eps <= 1
+            - basis.size() > 0
             - kernel_type is a kernel function object as defined in dlib/svm/kernel_abstract.h 
               It must be capable of operating on the elements of samples and basis.
             - vect1_type == a matrix or something convertible to a matrix via vector_to_matrix()