make certain that *_freq flags are initialized. use -1 instead of 0 to avoid division by zero

src/fix.cpp

@@ -81,6 +81,7 @@ Fix::Fix(LAMMPS *lmp, int /*narg*/, char **arg) :
 
   scalar_flag = vector_flag = array_flag = 0;
   peratom_flag = local_flag = 0;
+  global_freq = local_freq = peratom_freq = -1;
   size_vector_variable = size_array_rows_variable = 0;
 
   comm_forward = comm_reverse = comm_border = 0;