[fix] checker: don't run the checker when uwsgi is not properly configured

Before this commit, even with the scheduler disabled, the checker was running
at least once for each uwsgi worker.

searx/shared/shared_uwsgi.py

@@ -61,3 +61,4 @@ def schedule(delay, func, *args):
     _last_signal += 1
     uwsgi.register_signal(signal_num, 'worker', sighandler)
     uwsgi.add_timer(signal_num, delay)
+    return True