Fix start_comp offset in tracer particle routines (AMReX-Codes#4098)

This makes the routines work as expected given the variable names
(currently, `ncomp` was being treated as `end_comp`)

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_TracerParticle_mod_K.H

@@ -134,7 +134,7 @@ void linear_interpolate_to_particle (const P& p,
         amrex::Real sz[] = {amrex::Real(1.0) - zint, zint};
 #endif
 
-        for (int comp = start_comp; comp < ncomp; ++comp) {
+        for (int comp = start_comp; comp < start_comp + ncomp; ++comp) {
             val[ctr] = ParticleReal(0.0);
 #if (AMREX_SPACEDIM > 2)
             for (int kk = 0; kk <=1; ++kk) {
@@ -378,7 +378,7 @@ void linear_interpolate_to_particle_z (const P& p,
                                                hint_ilojhi,
                                                hint_ihijhi};
 #endif
-        for (int comp = start_comp; comp < ncomp; ++comp) {
+        for (int comp = start_comp; comp < start_comp + ncomp; ++comp) {
             val[ctr] = amrex::ParticleReal(0.);
 #if (AMREX_SPACEDIM == 2)
             int k0 = 0;
@@ -550,7 +550,7 @@ void linear_interpolate_to_particle_mapped (const P& p,
 
     int i = p.idata(0);
     int j = p.idata(1);
-    for (int comp = start_comp; comp < ncomp; ++comp) {
+    for (int comp = start_comp; comp < start_comp + ncomp; ++comp) {
 
 #if (AMREX_SPACEDIM == 2)
         // Value of data at surrounding nodes