tethering: Tolerate socket exception from ifindex look-up

The code already handles the case of internalNetworkInterface being null,
but this would never happen since it throws an exception if
NetworkInterface.getByName(internalInterface) fails. Catch the exception
and continue processing.

This is needed with some older usb gadget code which don't populate
/sys/class/net/usb0/.

Change-Id: I15adead7835ff3a4f77f87175ac85c5d9f45f9c0