Add a missing check for null pointer.

There is a race condition when processing intents inside
PairingCancelHandler.onReceive() that could trigger NPE.
A speculative list of events (untested) that could trigger it:

1. Trigger Pairing Cancel for a Bluetooth device.
2. Turn Off Bluetooth
   This should call CachedBluetoothDeviceManager.onBluetoothStateChanged()
   and should remove the device from (1) from the list of cached devices
3. BluetoothEventManager.PairincCancelHander.onReceive() receives
   an intent about about Pairing Cancel event.
   Within the processing of that intent, cachedDevice.getName()
   is called without checking whether cachedDevice is null.
   In this specific example, cachedDevice could be null because of (2)

Bug: 21368124
Change-Id: I86f5d5287b440d1d2e0fe147278b1c2257902e95