prevent call to get_bounds if no particles present

src/Algorithms/ParallelTTracker.cpp

@@ -599,8 +599,9 @@ void ParallelTTracker::computeExternalFields(OrbitThreader &oth) {
     Inform msg("ParallelTTracker ", *gmsg);
     const unsigned int localNum = itsBunch_m->getLocalNum();
     bool locPartOutOfBounds = false, globPartOutOfBounds = false;
-    Vector_t rmin, rmax;
-    itsBunch_m->get_bounds(rmin, rmax);
+    Vector_t rmin(0.0), rmax(0.0);
+    if (itsBunch_m->getTotalNum() > 0)
+        itsBunch_m->get_bounds(rmin, rmax);
     IndexMap::value_t elements;
 
     try {