Fix missing next check update causing the scheduler to execute checks too often

Regression from #6217, only in git master.

fixes #6421

lib/icinga/checkable-check.cpp

@@ -438,6 +438,12 @@ void Checkable::ExecuteCheck()
 	double scheduled_start = GetNextCheck();
 	double before_check = Utility::GetTime();
 
+	/* This calls SetNextCheck() which updates the CheckerComponent's idle/pending
+	 * queues and ensures that checks are not fired multiple times. ProcessCheckResult()
+	 * is called too late. See #6421.
+	 */
+	UpdateNextCheck();
+
 	bool reachable = IsReachable();
 
 	{