Fix null pointer

ConcurrentHashMap does not allow null to be used as a key or value.

> Exception occured in thread Thread[main,5,main] : 
> java.lang.RuntimeException: Unable to stop activity {net.osmand.plus/net.osmand.plus.activities.MapActivity}: java.lang.NullPointerException
> 	at android.app.ActivityThread.handleSleeping(ActivityThread.java:3461)
> 	at android.app.ActivityThread.access$2900(ActivityThread.java:163)
> 	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1445)
> 	at android.os.Handler.dispatchMessage(Handler.java:102)
> 	at android.os.Looper.loop(Looper.java:157)
> 	at android.app.ActivityThread.main(ActivityThread.java:5335)
> 	at java.lang.reflect.Method.invokeNative(Native Method)
> 	at java.lang.reflect.Method.invoke(Method.java:515)
> 	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265)
> 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081)
> 	at dalvik.system.NativeStart.main(Native Method)
> Caused by: java.lang.NullPointerException
> 	at java.util.concurrent.ConcurrentHashMap.put(ConcurrentHashMap.java:1017)
> 	at net.osmand.plus.routing.VoiceRouter.addVoiceMessageListener(VoiceRouter.java:845)
> 	at net.osmand.plus.helpers.WakeLockHelper.onStop(WakeLockHelper.java:79)
> 	at net.osmand.plus.activities.MapActivity.onStop(MapActivity.java:592)
> 	at android.app.Instrumentation.callActivityOnStop(Instrumentation.java:1230)
> 	at android.app.Activity.performStop(Activity.java:5534)
> 	at android.app.ActivityThread.handleSleeping(ActivityThread.java:3458)
> 	... 10 more