Clear the boundary particle indices' container before updating it. (AMReX-Codes#2907)

This avoids potential segmentation faults when one grid's particles all
move to other grids.

Author: hengjiew

Src/Particle/AMReX_NeighborParticlesI.H

@@ -821,6 +821,11 @@ selectActualNeighbors (CheckPair&& check_pair, int num_cells)
 
     for (int lev = 0; lev < this->numLevels(); ++lev)
     {
+        // clear previous neighbor particle ids
+        for (auto& keyval: m_boundary_particle_ids[lev]) {
+          keyval.second.clear();
+        }
+
         for (MyParIter pti(*this, lev); pti.isValid(); ++pti) {
             PairIndex index(pti.index(), pti.LocalTileIndex());