set wallet service to nsnull in case later on one of the other GetServices fail

in which case we 'goto done' and then try to release the wallet service... which
was never gotten  -doh!

xpfe/bootstrap/nsAppRunner.cpp

@@ -178,7 +178,7 @@ int main1(int argc, char* argv[])
   nsIDOMAppCoresManager *appCoresManager = nsnull;
   nsIURI* url = nsnull;
   nsIPref *prefs = nsnull;
-  nsIWalletService *walletService;
+  nsIWalletService *walletService = nsnull;
 
 #ifdef NECKO
   nsICookieService *cookieService = nsnull;