elementary: refactor elm_atspi_bridge function

elementary: refactor elm_atspi_bridge function

In order to avoid the following build warning:

lib/elementary/elm_atspi_bridge.c: In function
  ‘_children_changed_signal_send’:
lib/elementary/elm_atspi_bridge.c:3971:4: warning: ‘idx’ may
  be used uninitialized in this function [-Wmaybe-uninitialized]
    _bridge_signal_send(data, event->object, ATSPI_DBUS_INTERFACE_EVENT_OBJECT

A little refactoring was done to function _children_changed_signal_send.
Actually I wasn't able to see a codepath to use idx unintialized,
but this way is more clear and avoid compiler possible confusion.