Prevent crash

While not using the stumbler, a dialog would still pop up saying that the stumbler
had crashed:

E/AndroidRuntime(25424): Caused by: java.lang.NullPointerException
E/AndroidRuntime(25424):        at org.mozilla.mozstumbler.service.StumblerService.onStartCommand(StumblerService.java:138)
E/AndroidRuntime(25424):        at android.app.ActivityThread.handleServiceArgs(ActivityThread.java:2729)
E/AndroidRuntime(25424):        ... 10 more

This is annoying to users, so should be stopped.
Merge pull request #673 from keverets/stop-crash

Prevent crash