AP_Common: Include altitude in the init check for a location

libraries/AP_Common/Location.h

@@ -112,7 +112,7 @@ public:
      */
     float line_path_proportion(const Location &point1, const Location &point2) const;
 
-    bool initialised() const { return (lat !=0 || lng != 0); }
+    bool initialised() const { return (lat !=0 || lng != 0 || alt != 0); }
 
 private:
     static AP_Terrain *_terrain;