Fix NPE during phone calls

Sometimes I was randomly getting this exception.  This should fix it.
```
java.lang.NullPointerException
at com.jadn.cc.services.ContentService$1.onCallStateChanged(ContentService.java:535)
at android.telephony.PhoneStateListener$2.handleMessage(PhoneStateListener.java:739)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:158)
at android.app.ActivityThread.main(ActivityThread.java:5751)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1083)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:850)
at dalvik.system.NativeStart.main(Native Method)
```