QCFSocketNotifier: initialize member variable to avoid crash

The function pointer "maybeCancelWaitForMoreEvents" was never
initialized to any value. This caused qt_mac_socket_callback
to call that "function" when it had a non-zero value, which caused
a crash under some circumstances.

Change-Id: I1d31be7b5b7c103fdc0204ffa6e9a5ecbb28d062
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>