The stats might become null between these two checks, when a tunnel
flips off, resulting in a null pointer dereference:
at com.wireguard.android.model.ObservableTunnel.getStatisticsAsync (ObservableTunnel.java:103)
at com.wireguard.android.fragment.TunnelDetailFragment.updateStats (TunnelDetailFragment.java:108)
at com.wireguard.android.fragment.TunnelDetailFragment.access$updateStats (TunnelDetailFragment.java:27)
at com.wireguard.android.fragment.TunnelDetailFragment$onResume$1.run (TunnelDetailFragment.java:74)
at java.util.TimerThread.mainLoop (TimerThread.java:562)
at java.util.TimerThread.run (TimerThread.java:512)
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>