AP_Common: add initialised() method to Location

Sometimes code paths are crossed when they shouldn't be.  This method
can help in the short term if a codepath is calling methods on a
Location when it is not initialised, but generally a vehicle should be
calling position_ok() and other methods rather than this one.

libraries/AP_Common/Location.cpp

@@ -103,7 +103,7 @@ Location::AltFrame Location::get_alt_frame() const
 bool Location::get_alt_cm(AltFrame desired_frame, int32_t &ret_alt_cm) const
 {
 #if CONFIG_HAL_BOARD == HAL_BOARD_SITL
-    if (lat == 0 && lng == 0) {
+    if (!initialised()) {
         AP_HAL::panic("Should not be called on invalid location");
     }
 #endif

libraries/AP_Common/Location.h

@@ -112,6 +112,8 @@ public:
      */
     float line_path_proportion(const Location &point1, const Location &point2) const;
 
+    bool initialised() const { return (lat !=0 || lng != 0); }
+
 private:
     static AP_Terrain *_terrain;