Test we have logind proxy before taking the lock

Before trying to take the inhibitor lock via logind we should
check we actually have the logind connection, because we are not
failing in init() if we could not get the logind proxy.

src/application.c

@@ -931,6 +931,10 @@ gbb_application_inhibitor_lock_take(GbbApplication *application)
     GUnixFDList *fds;
     GError *error = NULL;
 
+    if (application->logind == NULL) {
+        return FALSE;
+    }
+
     if (application->inhibitor_fd > -1) {
         return TRUE;
     }