ObservableTunnel: account for race in renulling stats

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>

ui/src/main/java/com/wireguard/android/model/ObservableTunnel.kt

@@ -93,14 +93,14 @@ class ObservableTunnel internal constructor(
     @get:Bindable
     var statistics: Statistics? = null
         get() {
-            if (field == null || field!!.isStale)
+            if (field == null || field?.isStale != false)
                 manager.getTunnelStatistics(this).whenComplete(ExceptionLoggers.E)
             return field
         }
         private set
 
     val statisticsAsync: CompletionStage<Statistics>
-        get() = if (statistics == null || statistics!!.isStale)
+        get() = if (statistics == null || statistics?.isStale != false)
             manager.getTunnelStatistics(this)
         else
             CompletableFuture.completedFuture(statistics)