Fix use after free in the CallbackToLogRedirector (#5918)

The heap-use-after-free vulnerability occurs in the
CallbackToLogRedirector function. During the process of logging,
a previously freed memory region is accessed, leading to a
use-after-free condition. This vulnerability stems from incorrect
memory management, specifically, freeing a log stream and then
attempting to access it later on.

This patch sets NULL value for The DefaultStream global pointer.

Co-authored-by: Kim Kulling <kimkulling@users.noreply.github.com>

code/Common/Assimp.cpp

@@ -416,6 +416,10 @@ ASSIMP_API aiReturn aiDetachLogStream(const aiLogStream *stream) {
     DefaultLogger::get()->detachStream(it->second);
     delete it->second;
 
+    if ((Assimp::LogStream *)stream->user == DefaultStream) {
+        DefaultStream = nullptr;
+    }
+
     gActiveLogStreams.erase(it);
 
     if (gActiveLogStreams.empty()) {