Check "nj > 0" at the caller

We do not want to call "evaluate_gravity" when "nj==0". The check for this is now made at the caller, not at the callee.

Author: HannoSpreeuw

lib/sapporo_light/sapporo.cpp

@@ -151,7 +151,9 @@ void sapporo::calc_firsthalf(int cluster_id,
     acc_j.clear();
     jrk_j.clear();
   }
-  evaluate_gravity(ni, nj);
+  if (nj > 0) {
+    evaluate_gravity(ni, nj);
+  }
 
 }