8319704: LogTagSet::set_output_level() should not accept NULL as LogOutput

Reviewed-by: jsjolen, dholmes

src/hotspot/share/logging/logOutputList.cpp

@@ -52,6 +52,7 @@ void LogOutputList::wait_until_no_readers() const {
 }
 
 void LogOutputList::set_output_level(LogOutput* output, LogLevelType level) {
+  assert(output != nullptr, "LogOutput is null");
   LogOutputNode* node = find(output);
   if (level == LogLevel::Off && node != nullptr) {
     remove_output(node);