Fixed a runtime bug in the distance_function's + and - operators which triggered when

distance_functions with no basis vectors in them were added or subtracted.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%404128

dlib/svm/function.h

@@ -523,6 +523,11 @@ namespace dlib
                 << "\n\t You can only add two distance_functions together if they use the same kernel."
                 );
 
+            if (alpha.size() == 0)
+                return rhs;
+            else if (rhs.alpha.size() == 0)
+                return *this;
+            else
                 return distance_function(join_cols(alpha, rhs.alpha),
                                         b + rhs.b + 2*trans(alpha)*kernel_matrix(kernel_function,basis_vectors,rhs.basis_vectors)*rhs.alpha,
                                         kernel_function,
@@ -539,6 +544,13 @@ namespace dlib
                 << "\n\t You can only subtract two distance_functions if they use the same kernel."
                 );
 
+            if (alpha.size() == 0 && rhs.alpha.size() == 0)
+                return distance_function(kernel_function);
+            else if (alpha.size() != 0 && rhs.alpha.size() == 0)
+                return *this;
+            else if (alpha.size() == 0 && rhs.alpha.size() != 0)
+                return -1*rhs;
+            else
                 return distance_function(join_cols(alpha, -rhs.alpha),
                                         b + rhs.b - 2*trans(alpha)*kernel_matrix(kernel_function,basis_vectors,rhs.basis_vectors)*rhs.alpha,
                                         kernel_function,