Plane: fixed setting of waypoint when not in AUTO

this fixes a bug where the next waypoint is set incorrectly if set via
MAVLink when not in AUTO

ArduPlane/commands_process.pde

@@ -23,7 +23,18 @@ void change_command(uint8_t cmd_index)
         next_nav_command.id = NO_COMMAND;
         non_nav_command_ID      = NO_COMMAND;
 
-        nav_command_index       = cmd_index - 1;
+        /*
+          if we are in AUTO then we need to set the nav_command_index
+          to one less than the requested index as
+          process_next_command() will increment the index before using
+          it. If not in AUTO then we just set the index as give.
+          Thanks to Michael Day for finding this!
+         */
+        if (control_mode == AUTO) {
+            nav_command_index       = cmd_index - 1;
+        } else {
+            nav_command_index       = cmd_index;
+        }
         g.command_index.set_and_save(cmd_index);
         update_commands();
     }