cgrates: don't divide MaxUsage for postpaid

Do not divde MaxUsage 0 and -1 values with 10000000 when postpaid is
used, otherwise we will get invalid values and the session is
immediately closed.

Thanks to TeoV (Teofil Voivozeanu) and DanB (Dan Bogos) for reporting
and testing

modules/cgrates/cgrates_acc.c

@@ -220,7 +220,9 @@ static int cgr_proc_start_acc_reply(struct cgr_conn *c, json_object *jobj,
 					json_object_get_type(jobj), json_object_to_json_string(jobj));
 			return -4;
 		}
-		val.n = json_object_get_int64(jobj) / 1000000000;
+		val.n = json_object_get_int64(jobj);
+		if (val.n != 0 && val.n != -1)
+			val.n /= 1000000000;
 	} else {
 		if (json_object_get_type(jobj) != json_type_int) {
 			LM_ERR("CGRateS returned a non-int type for InitiateSession reply: %d %s\n",