Fix for Bug project-chip#36732 (project-chip#36879)

Set the app_state callback object in the Connection state to null
when the CASE session object is being cleared, on top of setting the
inner callback methods to null.
This prevents the callback object from being accessed later, when the
connection is getting closed(after the CASE session has been set up and
the session object no longer exists).

src/protocols/secure_channel/CASESession.cpp

@@ -428,12 +428,20 @@ void CASESession::Clear()
     mTCPConnCbCtxt.connClosedCb   = nullptr;
     mTCPConnCbCtxt.connReceivedCb = nullptr;
 
-    if (mPeerConnState && mPeerConnState->mConnectionState != Transport::TCPState::kConnected)
+    if (mPeerConnState)
     {
-        // Abort the connection if the CASESession is being destroyed and the
-        // connection is in the middle of being set up.
-        mSessionManager->TCPDisconnect(mPeerConnState, /* shouldAbort = */ true);
-        mPeerConnState = nullptr;
+        // Set the app state callback object in the Connection state to null
+        // to prevent any dangling pointer to memory(mTCPConnCbCtxt) owned
+        // by the CASESession object, that is now getting cleared.
+        mPeerConnState->mAppState = nullptr;
+
+        if (mPeerConnState->mConnectionState != Transport::TCPState::kConnected)
+        {
+            // Abort the connection if the CASESession is being destroyed and the
+            // connection is in the middle of being set up.
+            mSessionManager->TCPDisconnect(mPeerConnState, /* shouldAbort = */ true);
+            mPeerConnState = nullptr;
+        }
     }
 #endif // INET_CONFIG_ENABLE_TCP_ENDPOINT
 }