move xhold checks caller-side

this also allows it to fall-back on do-nothing rather than crash

src/MISC/fix_ipi.cpp

@@ -379,6 +379,7 @@ void FixIPI::initial_integrate(int /*vflag*/)
   // snapshot at neighbor list creation, minimizing the
   // number of neighbor list updates
   auto xhold = neighbor->get_xhold();
+  if (xhold != NULL) { // don't wrap if xhold is not used in the NL
     for (int i = 0; i < nlocal; i++) {
       if (mask[i] & groupbit) {
         auto delx = x[i][0] - xhold[i][0];
@@ -392,7 +393,7 @@ void FixIPI::initial_integrate(int /*vflag*/)
         x[i][2] = xhold[i][2] + delz;
       }
     }
-
+  }
   // move atoms to new processors via irregular()
   // only needed if migrate_check() says an atom moves to far
   if (domain->triclinic) domain->x2lamda(atom->nlocal);

src/neighbor.cpp

@@ -2976,13 +2976,11 @@ bigint Neighbor::get_nneigh_half()
   return nneighhalf;
 }
 
+/* ----------------------------------------------------------------------
+ return the pointer containing the last positions stored by the NL builder
+------------------------------------------------------------------------- */
 double **Neighbor::get_xhold()
 {
-  // Returns the pointer containing the last positions stored by the NL builder,
-  // checking it has actually been initialized
-  if (maxhold == 0) {
-    error->all(FLERR, "trying to access uninitialized xhold list");
-  }
   return xhold;
 }