AP_Rally: add is_valid method

method will be used in Copter to check if a rally point is inside fence

libraries/AP_Rally/AP_Rally.cpp

@@ -132,7 +132,7 @@ bool AP_Rally::find_nearest_rally_point(const Location &current_loc, RallyLocati
         Location rally_loc = rally_location_to_location(next_rally);
         float dis = get_distance(current_loc, rally_loc);
 
-        if (dis < min_dis || min_dis < 0) {
+        if (is_valid(rally_loc) && (dis < min_dis || min_dis < 0)) {
             min_dis = dis;
             return_loc = next_rally;
         }

libraries/AP_Rally/AP_Rally.h

@@ -62,6 +62,8 @@ public:
     static const struct AP_Param::GroupInfo var_info[];
 
 private:
+    virtual bool is_valid(const Location &rally_point) const { return true; }
+
     static StorageAccess _storage;
 
     // internal variables