PanService: Check mNetworkFactory for null

If tethering was never started, mNetworkFactory may be null.

Bug: 152379488
Test: 1. Pair DUT with remote mobile phone.
      2. Go to Bluetooth settings page of remote mobile phone
         and forget DUT.
      3. Try to reconnect DUT to remote mobile phone.
Change-Id: I5e8e7f2a0e5e191fcc2682c8faeec816a6aa7c5b