Plane: fix LOITER_TO_ALT to verify headings towards waypoints within the loiter radius.

Whenever next waypoint is within the loiter radius, maintaining loiter would prevent us from ever pointing toward the next waypoint. Hence for very close waypoints loiter_to_alt becomes verified by the altitude only.

ArduPlane/commands_logic.cpp

@@ -588,7 +588,15 @@ bool Plane::verify_loiter_to_alt()
                                        next_nav_cmd)) {
             //no next waypoint to shoot for -- go ahead and break out of loiter
             return true;        
-        } 
+        }
+
+        if (get_distance(next_WP_loc, next_nav_cmd.content.location) < labs(g.loiter_radius)) {
+            /* Whenever next waypoint is within the loiter radius, 
+               maintaining loiter would prevent us from ever pointing toward the next waypoint.
+               Hence break out of loiter immediately
+             */
+            return true;
+        }
 
         // Bearing in radians
         int32_t bearing_cd = get_bearing_cd(current_loc,next_nav_cmd.content.location);