AC_Fence: make member pointer to AP_InertialNav const, remove unused

gps-pointer member

libraries/AC_Fence/AC_Fence.cpp

@@ -55,7 +55,7 @@ const AP_Param::GroupInfo AC_Fence::var_info[] PROGMEM = {
 };
 
 /// Default constructor.
-AC_Fence::AC_Fence(AP_InertialNav* inav) :
+AC_Fence::AC_Fence(const AP_InertialNav* inav) :
     _inav(inav),
     _alt_max_backup(0),
     _circle_radius_backup(0),
@@ -232,4 +232,4 @@ float AC_Fence::get_breach_distance(uint8_t fence_type) const
 
     // we don't recognise the fence type so just return 0
     return 0;
-}
+}

libraries/AC_Fence/AC_Fence.h

@@ -32,7 +32,7 @@ class AC_Fence
 public:
 
     /// Constructor
-    AC_Fence(AP_InertialNav* inav);
+    AC_Fence(const AP_InertialNav* inav);
 
     /// enable - allows fence to be enabled/disabled.  Note: this does not update the eeprom saved value
     void enable(bool true_false) { _enabled = true_false; }
@@ -89,7 +89,7 @@ private:
     void clear_breach(uint8_t fence_type);
 
     // pointers to other objects we depend upon
-    AP_InertialNav* _inav;
+    const AP_InertialNav *const _inav;
 
     // parameters
     AP_Int8         _enabled;               // top level enable/disable control