Fix a crash in QBluetoothSocket on reconnection attempt (BlueZ)

Ensure a native socket is obtained after QBluetoothSocket has been closed.

[ChangeLog][QtBluetooth][QBluetoothSocket] Fixed a crash on reconnect.

Change-Id: I9f470d23366250dc20ad3305ddec16300c1ad8ec
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>