Particle Copy Plan: Default Vals (#3729)

## Summary

Valgrind finds those to be uninitialized in GPU runs with 1 MPI rank.

## Additional background

## Checklist

The proposed changes:
- [x] fix a bug or incorrect behavior in AMReX
- [ ] add new capabilities to AMReX
- [ ] changes answers in the test suite to more than roundoff level
- [ ] are likely to significantly affect the results of downstream AMReX
users
- [ ] include documentation in the code and/or rst files, if appropriate

Src/Particle/AMReX_ParticleCommunication.H

@@ -91,8 +91,8 @@ struct ParticleCopyPlan
     Vector<int> m_rcv_box_pids;
     Vector<int> m_rcv_box_levs;
 
-    Long m_NumSnds;
-    int m_nrcvs;
+    Long m_NumSnds = 0;
+    int m_nrcvs = 0;
     mutable Vector<MPI_Status> m_build_stats;
     mutable Vector<MPI_Request> m_build_rreqs;