Fixed bug in DefaultLogger::set: (#5826)

assigning s_pNullLogger to the parameter "logger" is definitely wrong.
However, custom logger previously set from user must not be deleted.
The user itself must handle allocation / deallocation.

code/Common/DefaultLogger.cpp

@@ -221,13 +221,11 @@ void DefaultLogger::set(Logger *logger) {
 #endif
 
     if (nullptr == logger) {
-        logger = &s_pNullLogger;
+        m_pLogger = &s_pNullLogger;
     }
-    if (nullptr != m_pLogger && !isNullLogger()) {
-        delete m_pLogger;
+    else {
+        m_pLogger = logger;
     }
-
-    DefaultLogger::m_pLogger = logger;
 }
 
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