OPP: Check target device when receiving SDP search result

* Return and print warning message when target is null while
  handling ACTION_SDP_RECORD
* Without this check, a NullPointerException could be triggered
  when a previously successful SDP search result landed on a
  BluetoothOppTransfer object will recently failed SDP search that would
  render mDevice null

Bug: 36260940
Test: make, test sending file through Bluetooth Opp
Change-Id: I861e7e64a7c9b316d4540fd5134892c247753f74
(cherry picked from commit 8e94b07a)