[fix] checker: fix engine statistics

Without this commit, the URL /stats/errors shows percentage above 100% after the checker has run.

searx/search/checker/impl.py

@@ -4,6 +4,7 @@ import typing
 import types
 import functools
 import itertools
+import threading
 from time import time
 from urllib.parse import urlparse
 
@@ -377,6 +378,8 @@ class Checker:
         engineref_category = search_query.engineref_list[0].category
         params = self.processor.get_params(search_query, engineref_category)
         if params is not None:
+            with threading.RLock():
+                self.processor.engine.stats['sent_search_count'] += 1
             self.processor.search(search_query.query, params, result_container, time(), 5)
         return result_container