Tethering: fix NPE when retrieving active interfaces

When in airplane mode getActiveNetworkInfo() may return null.

Change-Id: Icba37abb51b9378c8af73d9f6587e04c588dd85e
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>
Merge "Tethering: fix NPE when retrieving active interfaces" into kitkat