Fixed a crashing bug in trunk Whisper when used with 1.4 router

When trunk Whisper is run with 1.4 IBR-DTN router, it crashes when the
contact list or message list windows are closed. This is because binding
the security connection fails. The bind exception is caught, but then
attempting to unbind it causes an error. Track the connection bind status
so Whisper does not attempt to unbind an unbound connection.
A better approach would be to simply check if mSecurityService is null instead of introducing a new variable which has to be updated all the time.