clean up temp vorticity array before interpolating

bfps/cpp/particles/abstract_particles_system.hpp

@@ -75,6 +75,7 @@ public:
                                    const field<rnumber, be, fc>& in_field) {
         static_assert(fc == THREE, "only THREE is supported for now");
         std::unique_ptr<real_number[]> extra_rhs(new real_number[getLocalNbParticles()*3]());
+        std::fill_n(extra_rhs.get(), 3*getLocalNbParticles(), 0);
         sample_compute_field(in_field, extra_rhs.get());
         completeLoopWithVorticity(dt, extra_rhs.get());
     }