Skip UBSAN error (division by zero)

Happens when calling `system.integrator.run(0)`. In that case the
average instantaneous pressure is undefined and can only be NaN.

src/core/npt.cpp

@@ -16,20 +16,27 @@
  * You should have received a copy of the GNU General Public License
  * along with this program. If not, see <http://www.gnu.org/licenses/>.
  */
-#include "npt.hpp"
+#include <cmath>
+
 #include "communication.hpp"
 #include "config.hpp"
 #include "errorhandling.hpp"
 #include "integrate.hpp"
+#include "npt.hpp"
 
 #ifdef NPT
 void synchronize_npt_state(int n_steps) {
  nptiso.invalidate_p_vel = false;
  MPI_Bcast(&nptiso.p_inst, 1, MPI_DOUBLE, 0, comm_cart);
  MPI_Bcast(&nptiso.p_diff, 1, MPI_DOUBLE, 0, comm_cart);
  MPI_Bcast(&nptiso.volume, 1, MPI_DOUBLE, 0, comm_cart);
- if (this_node == 0)
- nptiso.p_inst_av /= 1.0 * n_steps;
+ if (this_node == 0) {
+ if (n_steps == 0) {
+ nptiso.p_inst_av = std::nan("");
+ } else {
+ nptiso.p_inst_av /= 1.0 * n_steps;
+ }
+ }
  MPI_Bcast(&nptiso.p_inst_av, 1, MPI_DOUBLE, 0, comm_cart);
 }