added internal fix dummy command to enable more control of fix ordering

src/fix.cpp

@@ -79,6 +79,7 @@ Fix::Fix(LAMMPS *lmp, int /*narg*/, char **arg) :
   respa_level = -1;
   maxexchange = 0;
   maxexchange_dynamic = 0;
+  pre_exchange_migrate = 0;
 
   scalar_flag = vector_flag = array_flag = 0;
   peratom_flag = local_flag = 0;