[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_simple.py

@@ -36,3 +36,4 @@ def schedule(delay, func, *args):
         func(*args)
 
     call_later()
+    return True