random failure in tests/world_builder_select_grains

[tjhei]

Error:

An error occurred in line <843> of file </ssd/aspect-git/source/simulator/simulator_access.cc> in function
    aspect::Particle::World<dim>& aspect::SimulatorAccess<dim>::get_particle_world(unsigned int) [with int dim = 3]
The violated condition was: 
    particle_world_index < simulator->particle_worlds.size()

[tjhei]

valgrind reports:

==215753== Conditional jump or move depends on uninitialised value(s)
==215753==    at 0x5EC70E4: aspect::SimulatorAccess<3>::get_particle_world(unsigned int) (simulator_access.cc:843)
==215753==    by 0x4A24531: aspect::Particle::Property::CpoElasticTensor<3>::initialize() (cpo_elastic_tensor.cc:69)
==215753==    by 0x4B201A7: aspect::Particle::Property::Manager<3>::initialize() (interface.cc:325)
==215753==    by 0x4B849F4: aspect::Particle::World<3>::parse_parameters(dealii::ParameterHandler&, unsigned int) (world.cc:1347)
==215753==    by 0x5CE3A4C: aspect::Simulator<3>::Simulator(ompi_communicator_t*, dealii::ParameterHandler&) (core.cc:457)
==215753==    by 0x3A391C6: void run_simulator<3>(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, bool, bool, bool, bool) (main.cc:606)
==215753==    by 0x39FF991: main (main.cc:818)

[tjhei]

fix incoming