Initialize m_is_valid when expensive preconditions are off!

Surface_mesh_parameterization/include/CGAL/Parameterization_mesh_patch_3.h

@@ -191,6 +191,7 @@ public:
         set_mesh_seaming(first_seam_vertex, end_seam_vertex);
 
         // Check that the cut mesh is 2-manifold
+        m_is_valid = true;
         CGAL_surface_mesh_parameterization_expensive_precondition_code( \
             m_is_valid = mesh.is_valid() && check_seam(first_seam_vertex, end_seam_vertex); \
         );