fixing a potential crash if prefs do not exist. need to set rv. npodb

extensions/spatialnavigation/src/nsSpatialNavigationService.cpp

@@ -105,7 +105,7 @@ nsSpatialNavigationService::Observe(nsISupports *aSubject, const char *aTopic, c
     NS_ENSURE_SUCCESS(rv, rv);
     windowWatcher->RegisterNotification(this);
     
-    nsCOMPtr<nsIPrefBranch2> prefBranch = do_GetService(NS_PREFSERVICE_CONTRACTID);
+    nsCOMPtr<nsIPrefBranch2> prefBranch = do_GetService(NS_PREFSERVICE_CONTRACTID, &rv);
     NS_ENSURE_SUCCESS(rv, rv);
     
 	prefBranch->AddObserver("snav.", this, PR_FALSE);
@@ -186,7 +186,6 @@ nsSpatialNavigationService::Observe(nsISupports *aSubject, const char *aTopic, c
 
 #define SpatialNavigationService_ContractID "@mozilla.org/spatialnavigation/service"
 
-
 static NS_METHOD SpatialNavigationServiceRegistration(nsIComponentManager *aCompMgr,
                                                       nsIFile *aPath,
                                                       const char *registryLocation,