NPE in BluetoothSocket.write()

If calling connect succeed, we should not see this NPE.
An IOException may happen after call BluetoothSocket.connect.
If you still call write after the IOException,
you will get this NPE.
add NPE protection for possible wrong calling sequence from application,
To make bluetoothSocket more error-tolerant.

bug:12104154
Change-Id: I7fa4e847b500ca9b9d2a43df432f31a1bb016c0a